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

chrome/common/extensions/api/cast_streaming_session.idl

Index: chrome/common/extensions/api/cast_streaming_session.idl
diff --git a/chrome/common/extensions/api/cast_streaming_session.idl b/chrome/common/extensions/api/cast_streaming_session.idl
index 6673baf0e170972b4517e989499b913cbcbe63aa..850e0dffb0f5c08ef1704722a554c48e499811c2 100644
--- a/chrome/common/extensions/api/cast_streaming_session.idl
+++ b/chrome/common/extensions/api/cast_streaming_session.idl
@@ -24,13 +24,16 @@ namespace cast.streaming.session {
     // 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.
+    // 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,
+        [instanceOf=MediaStreamTrack] optional object audioTrack,
+        [instanceOf=MediaStreamTrack] optional object videoTrack,
         CreateCallback callback);
   };
 };