Backout b51c3d819364 (bug 806736) for frequent intermittent failures in test_privbrowsing_perwindowpb.html
Bug 826879 - Remove a strict aliasing footgun from MutableHandle; r=bhackett

These are unused and could be a strict aliasing hazard in the future.

--HG--
extra : rebase_source : c544d9abe3c0a8e8744dabcd7357a345e02ae120