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

Source/core/html/track/AudioTrack.idl

Index: Source/core/html/track/AudioTrack.idl
diff --git a/Source/core/html/track/AudioTrack.idl b/Source/core/html/track/AudioTrack.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 AudioTrack {
+  readonly attribute DOMString id;
+  readonly attribute DOMString kind;
+  readonly attribute DOMString label;
+  readonly attribute DOMString language;
+  attribute boolean enabled;
+};