Create and use a rigorous way to go from Window -> WindowProxy

[domenic]

As noted in #7348 (comment), we sometimes say something vague like "the WindowProxy that corresponds to window", and sometimes we do something rigorous-but-hard-to-understand, like "window's relevant Realm.[[GlobalEnv]].[[GlobalThisValue]]" or "window's relevant Realm.[[GlobalEnv]]'s EnvironmentRecord's [[GlobalThisValue]]".

We should create a wrapper definition, e.g. "window's corresponding WindowProxy", and use that throughout.