Bug 852164: Adds an addiitonal check to ensure that Plugin Hang UI user response doesn't race past a cancellation. r=bsmedberg

--HG--
extra : rebase_source : c626664d64183a00a9163202eaaa221c4daee260
Bug 843641 - Ensure tabs button images are refreshed on device rotation (r=mfinkle)
Bug 850936 - Actually update toolbar layout state on rotation (r=mfinkle)