The intent is to use these to autogenerate prototype declarations for
Window and WorkerGlobalScope classes.
And the spec links are just nice to have :^)
Specifically HTMLIFrameElement and HTMLObjectElement. HTMLEmbedElement
will gain it automatically once it's also converted to inherit from
BrowsingContextContainer.
This is no longer needed as BrowsingContextContainer::content_document()
now does the right thing, and HTMLIFrameElement.contentDocument is the
only user of this attribute. Let's not invent our own mechanisms for
things that are important to get right, like same origin comparisons.
