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.
Rick Eyre
Ralph Giles