Unify the task source and task canceller API

I moved away from the `Window` struct all the logic to handle task
sources, into a new struct called `TaskManager`. In a happy world, I'd
be able to just have there two functions, of the types:

```rust
fn task_source<T: TaskSource>(&self, name: TaskSourceName) -> Box<T>
fn task_source_with_canceller<T: TaskSource>(&self, name: TaskSourceName)
  -> (Box<T>, TaskSourceCanceller)
```

And not so much duplicated code. However, because TaskSource can't be a
trait object (because it has generic type parameters), that's not
possible. Instead, I decided to reduce duplicated logic through macros.

For reasons[1], I have to pass both the name of the function with
canceller and the name of the function without, as I'm not able to
concatenate them in the macro itself. I could probably use
`concat_idents` to create both types already defined and reduce the
amount of arguments by one, but that macro is nightly only. At the same
time, not being able to declare macros inside `impl` forces me to pass
`self` as an argument.

All this makes this solution more verbose than it would be ideally. It
does reduce duplication, but it doesn't reduce the size of the file.

[1](https://github.com/rust-lang/rust/issues/29599)

components/script/task_manager.rs

@@ -0,0 +1,162 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::dom::bindings::cell::DomRefCell;
+use crate::task::TaskCanceller;
+use crate::task_source::dom_manipulation::DOMManipulationTaskSource;
+use crate::task_source::file_reading::FileReadingTaskSource;
+use crate::task_source::history_traversal::HistoryTraversalTaskSource;
+use crate::task_source::media_element::MediaElementTaskSource;
+use crate::task_source::networking::NetworkingTaskSource;
+use crate::task_source::performance_timeline::PerformanceTimelineTaskSource;
+use crate::task_source::remote_event::RemoteEventTaskSource;
+use crate::task_source::user_interaction::UserInteractionTaskSource;
+use crate::task_source::websocket::WebsocketTaskSource;
+use crate::task_source::TaskSourceName;
+use std::collections::HashMap;
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+
+macro_rules! task_source_functions {
+    ($self:ident,$with_canceller:ident,$task_source:ident,$task_source_type:ident,$task_source_name:ident) => {
+        pub fn $with_canceller(&$self) -> ($task_source_type, TaskCanceller) {
+            ($self.$task_source.clone(), $self.task_canceller(TaskSourceName::$task_source_name))
+        }
+
+        pub fn $task_source(&$self) -> $task_source_type {
+            $self.$task_source.clone()
+        }
+    }
+}
+
+#[derive(JSTraceable, MallocSizeOf)]
+pub struct TaskManager {
+    #[ignore_malloc_size_of = "task sources are hard"]
+    pub task_cancellers: DomRefCell<HashMap<TaskSourceName, Arc<AtomicBool>>>,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    dom_manipulation_task_source: DOMManipulationTaskSource,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    file_reading_task_source: FileReadingTaskSource,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    history_traversal_task_source: HistoryTraversalTaskSource,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    media_element_task_source: MediaElementTaskSource,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    networking_task_source: NetworkingTaskSource,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    performance_timeline_task_source: PerformanceTimelineTaskSource,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    user_interaction_task_source: UserInteractionTaskSource,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    remote_event_task_source: RemoteEventTaskSource,
+    #[ignore_malloc_size_of = "task sources are hard"]
+    websocket_task_source: WebsocketTaskSource,
+}
+
+impl TaskManager {
+    pub fn new(
+        dom_manipulation_task_source: DOMManipulationTaskSource,
+        file_reading_task_source: FileReadingTaskSource,
+        history_traversal_task_source: HistoryTraversalTaskSource,
+        media_element_task_source: MediaElementTaskSource,
+        networking_task_source: NetworkingTaskSource,
+        performance_timeline_task_source: PerformanceTimelineTaskSource,
+        user_interaction_task_source: UserInteractionTaskSource,
+        remote_event_task_source: RemoteEventTaskSource,
+        websocket_task_source: WebsocketTaskSource,
+    ) -> Self {
+        TaskManager {
+            dom_manipulation_task_source,
+            file_reading_task_source,
+            history_traversal_task_source,
+            media_element_task_source,
+            networking_task_source,
+            performance_timeline_task_source,
+            user_interaction_task_source,
+            remote_event_task_source,
+            websocket_task_source,
+            task_cancellers: Default::default(),
+        }
+    }
+
+    task_source_functions!(
+        self,
+        dom_manipulation_task_source_with_canceller,
+        dom_manipulation_task_source,
+        DOMManipulationTaskSource,
+        DOMManipulation
+    );
+
+    task_source_functions!(
+        self,
+        media_element_task_source_with_canceller,
+        media_element_task_source,
+        MediaElementTaskSource,
+        MediaElement
+    );
+
+    task_source_functions!(
+        self,
+        user_interaction_task_source_with_canceller,
+        user_interaction_task_source,
+        UserInteractionTaskSource,
+        UserInteraction
+    );
+
+    task_source_functions!(
+        self,
+        networking_task_source_with_canceller,
+        networking_task_source,
+        NetworkingTaskSource,
+        Networking
+    );
+
+    task_source_functions!(
+        self,
+        file_reading_task_source_with_canceller,
+        file_reading_task_source,
+        FileReadingTaskSource,
+        FileReading
+    );
+
+    task_source_functions!(
+        self,
+        history_traversal_task_source_with_canceller,
+        history_traversal_task_source,
+        HistoryTraversalTaskSource,
+        HistoryTraversal
+    );
+
+    task_source_functions!(
+        self,
+        performance_timeline_task_source_with_canceller,
+        performance_timeline_task_source,
+        PerformanceTimelineTaskSource,
+        PerformanceTimeline
+    );
+
+    task_source_functions!(
+        self,
+        remote_event_task_source_with_canceller,
+        remote_event_task_source,
+        RemoteEventTaskSource,
+        RemoteEvent
+    );
+
+    task_source_functions!(
+        self,
+        websocket_task_source_with_canceller,
+        websocket_task_source,
+        WebsocketTaskSource,
+        Websocket
+    );
+
+    pub fn task_canceller(&self, name: TaskSourceName) -> TaskCanceller {
+        let mut flags = self.task_cancellers.borrow_mut();
+        let cancel_flag = flags.entry(name).or_insert(Default::default());
+        TaskCanceller {
+            cancelled: Some(cancel_flag.clone()),
+        }
+    }
+}