Bug 833283 - Add do_QueryFrame support to nsVideoFrame. r=bz

Ms2ger recommended this instead of using do_QueryInterface on
nsVideoFrame.
Bug 833382 - Implement WebVTTLoadListener r=cpearce,bz

- Implemented WebVTTLoadListener to manage the webvtt parser.
- TextTrackCue now handles the conversion of webvtt nodes to
  anonymous content which will be displayed on the video div
  overlay.
- HTMLTrackElement manages the lifetime of the WebVTTLoadListener.