[heycam/webidl] Review handling of WindowProxy (#656)

@bzbarsky opens this can of worms in https://github.com/heycam/webidl/pull/654#discussion_r259869833.

Probably every instance of "implements" or "is a platform object" should allow WindowProxy.

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/656