script: Move TaskManager to GlobalScope (#34827)

This is a simplification of the internal `TaskQueue` API that moves the
`TaskManager` to the `GlobalScope` itself. In addition, the handling of
cancellers is moved to the `TaskManager` as well. This means that no
arguments other than the `task` are necessary for queueing tasks, which
makes the API a lot easier to use and cleaner.

`TaskSource` now also keeps a copy of the canceller with it, so that
they always know the proper way to cancel any tasks queued on them.

There is one complication here. The event loop `sender` for dedicated
workers is constantly changing as it is set to `None` when not handling
messages. This is because this sender keeps a handle to the main
thread's `Worker` object, preventing garbage collection while any
messages are still in flight or being handled. This change allows
setting the `sender` on the `TaskManager` to `None` to allow proper
garbabge collection.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/task.rs

@@ -67,15 +67,15 @@ impl fmt::Debug for dyn TaskBox {
 }
 
 /// Encapsulated state required to create cancellable tasks from non-script threads.
-#[derive(Clone)]
+#[derive(Clone, Default, JSTraceable, MallocSizeOf)]
 pub struct TaskCanceller {
+    #[ignore_malloc_size_of = "This is difficult, because only one of them should be measured"]
     pub cancelled: Arc<AtomicBool>,
 }
 
 impl TaskCanceller {
-    /// Returns a wrapped `task` that will be cancelled if the `TaskCanceller`
-    /// says so.
-    pub fn wrap_task<T>(&self, task: T) -> impl TaskOnce
+    /// Returns a wrapped `task` that will be cancelled if the `TaskCanceller` says so.
+    pub(crate) fn wrap_task<T>(&self, task: T) -> impl TaskOnce
     where
         T: TaskOnce,
     {