Bug 905405. Wrap mLastNativeEventScheduled in a mutex to avoid racing on it. r=bsmedberg

--HG--
extra : rebase_source : 769e8870f4bf33407d01d944d8969459b9a161ce
Bug 913438. Remove mAnchor check since it doesn't help correct inconsistency between ImageLayer snapping and regular snapped image drawing, and can hurt performance. r=tnikkel

--HG--
extra : rebase_source : 5f62e052d20a9acb0a428b988e2d9b16c7b19dbc