Add the OnAddTrack callback for Objective-C wrapper.

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

 /*
  *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 88 matching lines @@
     didGenerateIceCandidate:(RTCIceCandidate *)candidate;
 
 /** Called when a group of local Ice candidates have been removed. */
 - (void)peerConnection:(RTCPeerConnection *)peerConnection
     didRemoveIceCandidates:(NSArray<RTCIceCandidate *> *)candidates;
 
 /** New data channel has been opened. */
 - (void)peerConnection:(RTCPeerConnection *)peerConnection
     didOpenDataChannel:(RTCDataChannel *)dataChannel;
 
+@optional
+/** Called when a new track is signaled from remote peer. The new track is
+ * attached to the RtpReceiver.
+ * TODO(zhihuang): Remove the @optional once the downstream applitions implement this method.*/
+- (void)peerConnection:(RTCPeerConnection *)peerConnection
+           didAddTrack:(RTCRtpReceiver *)rtpReceiver
+               streams:(NSArray<RTCMediaStream *> *)mediaStreams;
+
 @end
 
 RTC_EXPORT
 @interface RTCPeerConnection : NSObject
 
 /** The object that will be notifed about events such as state changes and
  *  streams being added or removed.
  */
 @property(nonatomic, weak, nullable) id<RTCPeerConnectionDelegate> delegate;
 @property(nonatomic, readonly) NSArray<RTCMediaStream *> *localStreams;
@@ skipping 96 matching lines @@
  */
 - (void)statsForTrack:
     (nullable RTCMediaStreamTrack *)mediaStreamTrack
      statsOutputLevel:(RTCStatsOutputLevel)statsOutputLevel
     completionHandler:
     (nullable void (^)(NSArray<RTCLegacyStatsReport *> *stats))completionHandler;
 
 @end
 
 NS_ASSUME_NONNULL_END