Bug 949642 - Implement VTTCue::LineAlign. r=rillian
Bug 882299 - Implement VTTCue::Line. r=smaug, r=rillian
Bug 921484 - Fix bugs found in TextTrack::GetActiveCues(). r=rillian
Bug 950830 - Clean up TextTrackCue style and TODO comments. r=rillian