Bug 865407 - Part 1: Add TextTrackManager class r=cpearce

This class will manage everything to do with TextTracks for
the HTMLMediaElement.
Bug 865407 - Part 2: Change TextTrackList::RemoveTextTrack() to receive a pointer.

The only code using this is TextTrackManager::RemoveTextTrack() which
receives a non-const pointer.
Bug 865407 - Part 3: Add TextTrackList::GetAllActiveCues().
Bug 865407 - Part 4: Add TextTrackList::GetAllRegions().
Bug 865407 - Part 5: Expose TextTrackCue::DisplayState to chrome JS.
Bug 865407 - Part 6: Move update display of cues code to TextTrackManager
Work.