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/dom/htmliframeelement.rs

@@ -237,7 +237,7 @@ impl HTMLIFrameElement {
             let this = Trusted::new(self);
             let pipeline_id = self.pipeline_id().unwrap();
             // FIXME(nox): Why are errors silenced here?
-            let _ = window.dom_manipulation_task_source().queue(
+            let _ = window.task_manager().dom_manipulation_task_source().queue(
                 task!(iframe_load_event_steps: move || {
                     this.root().iframe_load_event_steps(pipeline_id);
                 }),