Added AudioTrack, AudioTrackList, VideoTrack, VideoTrackList, and TrackEvent interfaces

components/script/dom/webidls/TrackEvent.webidl

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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://html.spec.whatwg.org/multipage/#the-trackevent-interface
+
+[Exposed=Window,
+ Constructor(DOMString type, optional TrackEventInit eventInitDict)]
+interface TrackEvent : Event {
+  readonly attribute (VideoTrack or AudioTrack or TextTrack)? track;
+};
+
+dictionary TrackEventInit : EventInit {
+  (VideoTrack or AudioTrack or TextTrack)? track = null;
+};