Bug 860338 - remove MOZ_WEBVTT.

The webvtt dom implementation and parser are a small amount of code.
Given the tendency of build-time switches to bitrot, it's not worth
keeping this one.
Bug 833385 - Implement HTMLTrackElement and TextTrack.

Update to version 9 of the patch from bugzilla.mozilla.org.

Also includes some of the removal of MOZ_MEDIA.