Add EventTarget to TextTrackList
According to the second paragraph of
TextTrackList should implement EventTarget so that one can listen for
'addtrack' and 'removetrack' events on the list.
@@ -2294,6 +2294,7 @@ Text.prototype.getDestinationInsertionPoints = function() {};
/**
* @see http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#texttracklist
* @constructor
+ * @implements {EventTarget}
* @implements {IArrayLike<!TextTrack>}
*/
function TextTrackList() {}
