Add the OnAddTrack callback for Objective-C wrapper.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h

Index: webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
diff --git a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
index 4438eb28ab5ecf4fb53c4a7e645ff0c08bd4c5da..147f6c95d3f99a6d799dbabb9988c3ba8c033b0b 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
+++ b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
@@ -106,6 +106,13 @@ RTC_EXPORT
 - (void)peerConnection:(RTCPeerConnection *)peerConnection
     didOpenDataChannel:(RTCDataChannel *)dataChannel;
 
+@optional

[tkchin_webrtc, 2017/02/15 23:43:48]

either all these methods should be optional or none of them

unless there's a specific reason this one needs to be optional?

[Zhi Huang, 2017/02/21 04:55:50]

I thought there would be at least some warnings if I add an non-optional method
in the protocol and the downstream application doesn't implement it. This could
be a temporary fix and I can leave a TODO here. Once the downstream apps add
this method, we can remove the "optional".

If it is OK to have some warnings or I'm just worrying too much about it, I can
remove the "@optional".

+/** Called when a new track is signaled from remote peer. The new track is
+ * attached to the RtpReceiver. */
+- (void)peerConnection:(RTCPeerConnection *)peerConnection
+           didAddTrack:(RTCRtpReceiver *)rtpReceiver
+       attachedStreams:(NSArray<RTCMediaStream *> *)mediaStreams;
+
 @end
 
 RTC_EXPORT