Bug 868611 - Implement HTMLTrackElement::UbindFromTree. r=rillian

Update HTMLTrackElement::UnbindFromTree to remove its TextTrack from
its HTMLMediaElement's list of TextTracks.
Bug 885205 - Extremely slow reflect bookmarks menu items after restoring bookmarks. And disk access abnormal. And Firefox process does not disappear from task manager on exit. r=mak
Bug 889123 - Implement Gecko preference for enabling of pre-installed applications. r=fabrice