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..ee822b6fcbf7ee47110afc1e3abf1ebb8d4e1a0c
--- /dev/null
+++ b/Source/modules/mediasource/TrackDefaultList.idl
@@ -0,0 +1,16 @@
+// 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 18 Nov 2014 Editor's Draft
+// https://w3c.github.io/media-source/#trackdefaultlist
+
+[
+    Constructor(sequence<TrackDefault> trackDefaults),
+    RaisesException=Constructor,
+    GarbageCollected,
+    RuntimeEnabled=MediaSourceExperimental,
+] interface TrackDefaultList {
+    readonly attribute unsigned long length;
+    [ImplementedAs=item] getter TrackDefault (unsigned long index);
+};