We should make sure we have the right ownership relationships among Document, Window, BrowsingContext, and window proxies.

Note that https://html.spec.whatwg.org/multipage/#garbage-collection-and-browsing-contexts says "A WindowProxy does not have a strong reference to the browsing context it was created alongside. In particular, it is possible for a nested browsing context to be discarded even if JavaScript code holds a reference to its WindowProxy object," which we should be careful about since we use BrowsingContext to stand in for WindowProxy.

Also, it is possible for a Document to have a strong reference to its browsing context, even after the context has been discarded. IRC chat with @domenic: http://logs.glob.uno/?c=freenode%23whatwg&s=1+Dec+2016&e=1+Dec+2016#c1013938.