Bug 912322 - Update semantics of IsChromeOrXBL to return true for remote XUL. r=bz

This brings us into alignment with nsContentUtils::IsCallerXBL(). We also take
the opportunity to clean up some comments and invariants that changed with the
removal of the XBL bit.
Bug 912322 - Fix tests. r=bz

The crashtest changes are untested (aside from the fact that they don't crash).
Bug 912322 - Stop making XBL methods available to the web. r=bz