Doing this will let us simplify the plumbing for frames / frame trees: right now, there's a lot of indirection to get from core/frame into WebLocalFrame (everything ends up getting plumbed through FrameLoaderClient / FrameClient, even if it's unrelated to loading).

Unfortunately, there are a number of obstacles to implementing this: for one, WebFrame / WebLocalFrame have a bunch of methods that use web types like WebNode. One potential solution is to split WebFrame into platform and keep WebLocalFrame in web, though that feels rather odd.