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

Source/core/html/track/AudioTrackList.idl

Index: Source/core/html/track/AudioTrackList.idl
diff --git a/Source/core/html/track/AudioTrackList.idl b/Source/core/html/track/AudioTrackList.idl
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+// 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.
+
+[
+    RuntimeEnabled=AudioVideoTracks,
+] interface AudioTrackList : EventTarget {
+  readonly attribute unsigned long length;
+  getter AudioTrack (unsigned long index);
+  AudioTrack? getTrackById(DOMString id);
+  
+  attribute EventHandler onchange;
+  attribute EventHandler onaddtrack;
+  attribute EventHandler onremovetrack;
+};