Re: [webcomponents] <iframe> and Window's named properties (#145)

So if an `<iframe>` works it becomes a "child browsing context" and it'll be "nested through" so we'll need to carefully vet everything that talks about that (and update it to account for shadow trees existing), including indeed the named properties on `Window`.

I think that for named properties it would make sense to not expose `<iframe>` from a shadow tree, whether it's open or closed. That would match what we do for `activeElement` and such. @smaug----, makes sense?

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