Cast: cast.streaming API to support null audio (or video) track

chrome/common/extensions/api/cast_streaming_session.idl

 // Copyright 2013 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.
 
 // The <code>chrome.cast.streaming.session</code> API creates a Cast
 // session using WebMediaStreamTrack as sources. The session is composed
 // by RTP streams and a network transport.
 //
 // Calling this API will generate corresponding resources for use with
 // chrome.cast.streaming.rtpStream and chrome.cast.streaming.udpTransport
 // APIs.
 //
 // Valid resource IDs are positive and non-zero.
 namespace cast.streaming.session {
   // Callback from the <code>create</code> method.
   // |audioStreamId| : The audio RTP stream ID.
   // |videoStreamId| : The video RTP stream ID.
   // |udpTransportId| : The UDP transport ID.
   callback CreateCallback = void (long audioStreamId,
                                   long videoStreamId,
                                   long udpTransportId);
 
   interface Functions {
     // Creates a Cast session using the provided audio and video track as
-    // source. This will create two RTP streams and a UDP transport that
-    // builds the session.
+    // source. The tracks must be of type MediaStreamTrack. This will
+    // create two RTP streams and a UDP transport that builds the session.
+    // Either |audioTrack| or |videoTrack| can be null but not both. This
+    // means creating a session with only audio or video. If a given
+    // track is null then the created stream ID will be null.
     //
     // |audioTrack| : the source audio track.
     // |videoTrack| : the source video track.
     // |callback| : Called when the sesion has been created.
-    [nocompile] static void create(
-        [instanceOf=MediaStreamTrack] object audioTrack,
-        [instanceOf=MediaStreamTrack] object videoTrack,
+    [nocompile,allowAmbiguousOptionalArguments] static void create(
+        [instanceOf=MediaStreamTrack] optional object audioTrack,
+        [instanceOf=MediaStreamTrack] optional object videoTrack,
         CreateCallback callback);
   };
 };