MSE: Implement TrackDefaultList object

Source/modules/mediasource/TrackDefaultList.idl

Index: Source/modules/mediasource/TrackDefaultList.idl
diff --git a/Source/modules/mediasource/TrackDefaultList.idl b/Source/modules/mediasource/TrackDefaultList.idl
new file mode 100644
index 0000000000000000000000000000000000000000..a5d2427908f403227ac1e2a52c08ce3644bd0b09
--- /dev/null
+++ b/Source/modules/mediasource/TrackDefaultList.idl
@@ -0,0 +1,17 @@
+// 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.
+
+// Per 06 Oct 2014 Editor's Draft
+// https://dvcs.w3.org/hg/html-media/raw-file/tip/media-source/media-source.html#idl-def-TrackDefaultList
+
+[
+    Constructor(sequence<TrackDefault> trackDefaults),
+    RaisesException=Constructor,
+    GarbageCollected,
+    RuntimeEnabled=MediaSourceExperimental,
+    TypeChecking=Interface,
+] interface TrackDefaultList {
+  readonly attribute unsigned long length;
+  getter TrackDefault item(unsigned long index);
+};