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/serviceworkerglobalscope.rs

@@ -412,7 +412,7 @@ impl ServiceWorkerGlobalScope {
                             }
                         },
                         reporter_name,
-                        scope.script_chan(),
+                        global.event_loop_sender(),
                         CommonScriptMsg::CollectReports,
                     );
 
@@ -485,7 +485,7 @@ impl ServiceWorkerGlobalScope {
         }
     }
 
-    pub fn script_chan(&self) -> Box<dyn ScriptChan + Send> {
+    pub(crate) fn event_loop_sender(&self) -> Box<dyn ScriptChan + Send> {
         Box::new(ServiceWorkerChan {
             sender: self.own_sender.clone(),
         })