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/dom/worker.rs

@@ -220,12 +220,15 @@ impl WorkerMethods<crate::DomTypeHolder> for Worker {
         let (control_sender, control_receiver) = unbounded();
         let (context_sender, context_receiver) = unbounded();
 
+        let event_loop_sender = global
+            .event_loop_sender()
+            .expect("Tried to create a worker in a worker while not handling a message?");
         let join_handle = DedicatedWorkerGlobalScope::run_worker_scope(
             init,
             worker_url,
             devtools_receiver,
             worker_ref,
-            global.script_chan(),
+            event_loop_sender,
             sender,
             receiver,
             worker_load_origin,