Add webidl interfaces and implementations of the HTML <track>
element and related TextTrack, TextTrackList, TextTrackCue,
and TextTrackCueList dom objects.
Visibility is controlled by the media.webvtt.enabled pref,
which defaults to false.
HTMLMediaElement:NewURIFromString() is hoisted to
nsGenericHTMLElement so it's available to the track
element as well.
This patch is primarily work by Dale Karp, David Humphrey
and others as Seneca College.
