[whatwg] Spec for location object needs to make some properties unforgeable; need supporting WebIDL changes

[Cameron McCormack]

Boris Zbarsky:
> I guess the question is whether we're more likely to need [Unforgeable]
> on some other entire interface or whether were more likely to need
> [Unforgeable] on a single member that's not a readonly attribute.  Of
> course we might never need either one...

I'm inclined to simplify now (by moving [Unforgeable] to the interface) 
and worry about it later if we need to.