Initial implementation of AudioTrack, AudioTrackList, VideoTrack, and VideoTrackList.

Source/core/html/track/VideoTrackList.idl

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+[
+    SetWrapperReferenceFrom=owner,
+    RuntimeEnabled=AudioVideoTracks
+] interface VideoTrackList : EventTarget {
+  readonly attribute unsigned long length;
+  getter VideoTrack (unsigned long index);
+  VideoTrack getTrackById(DOMString id);
+  readonly attribute long selectedIndex;
+
+  attribute EventHandler onchange;
+  attribute EventHandler onaddtrack;
+  attribute EventHandler onremovetrack;
+};