Implement AnimationFrameProvider for DedicatedWorkerGlobalScope

[gterzian]

Spec: https://html.spec.whatwg.org/multipage/imagebitmap-and-animations.html#animationframeprovider

See the Window webidl for an example:

servo/components/script/dom/webidls/Window.webidl

Line 63 in 6ca62aa

unsigned long requestAnimationFrame(FrameRequestCallback callback);

Relevant once we have #24276

[gterzian]

The interesting question is how to communicate between the compositor and a dedicated worker to trigger the animation callbacks.

Currently the constellation is not aware of such workers, which are "hidden" behind an EventLoop. So the constellation can only send messages to a ScriptThread, via the EventLoop interface.

We could have the script-thread forward the messages to the worker, and that might defeat the purpose of having the worker operate on an offscreen canvas independently from the "potentially janky JS main-thread"(effectively the script-thread), so we might want to add some plumbing so the worker can receive messages from the constellation/compositor directly and the rendering workflow is isolated from the script-thread.

I guess that might also mean having a direct sender from the worker to the relevant layout thread to trigger reflows? Or is that not relevant if the worker is only manipulating an OffscreenCanvas?