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/webglquery.rs

@@ -171,12 +171,10 @@ impl WebGLQuery {
                 this.update_results(&context);
             });
 
-            let global = self.global();
-            global
-                .as_window()
+            self.global()
                 .task_manager()
                 .dom_manipulation_task_source()
-                .queue(task, global.upcast())
+                .queue(task)
                 .unwrap();
         }