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 r=rillian

The only code using this is TextTrackManager::RemoveTextTrack() which
receives a non-const pointer.