Implement 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
new file mode 100644
index 0000000000000000000000000000000000000000..caf4046ad8e494f35bb85abe98793134457edd56
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+++ b/Source/core/html/track/AudioTrackList.idl
@@ -0,0 +1,15 @@
+// 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,

[sof, 2014/05/22 11:47:36]

Add [WillBeGarbageCollected]

[philipj_slow, 2014/05/22 12:51:02]

As discussed on IRC with sof, this is apparently not needed:

12:42 <@philipj> and is it an oversight that TextTrackList.idl did not get
[WillBeGarbageCollected]?
12:42 < sof> hmm..wait a sec
12:44 < sof> ah, TextTrackList is an EventTarget, which is a hybrid
refcounted-GC thing currently.
12:45 <@philipj> ah, but so is AudioTrackList and VideoTrackList, so... what
does that mean, exactly?
12:45 < sof> so you don't need [WillBeGarbageCollected] on the *List interfaces,
just "leaf" track objects here.

[sof, 2014/05/22 12:51:56]

Sorry, let's leave this EventTarget-derived interface without
[WillBeGarbageCollected]. When EventTarget switches, this will automatically get
it also.

This applies to VideoTrackList.idl also; sorry about the confusion.

+] 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;
+};