Add the OnAddTrack callback for Objective-C wrapper.

webrtc/sdk/objc/Framework/Classes/RTCRtpReceiver.mm

 /*
  *  Copyright 2016 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 54 matching lines @@
     return [[RTCMediaStreamTrack alloc] initWithNativeTrack:nativeTrack];
   }
   return nil;
 }
 
 - (NSString *)description {
   return [NSString stringWithFormat:@"RTCRtpReceiver {\n  receiverId: %@\n}",
       self.receiverId];
 }
 
+- (void)dealloc {
+    _nativeRtpReceiver->SetObserver(nullptr);
+}
+
 - (BOOL)isEqual:(id)object {
   if (self == object) {
     return YES;
   }
   if (object == nil) {
     return NO;
   }
   if (![object isMemberOfClass:[self class]]) {
     return NO;
   }
@@ skipping 29 matching lines @@
     case cricket::MEDIA_TYPE_AUDIO:
       return RTCRtpMediaTypeAudio;
     case cricket::MEDIA_TYPE_VIDEO:
       return RTCRtpMediaTypeVideo;
     case cricket::MEDIA_TYPE_DATA:
       return RTCRtpMediaTypeData;
   }
 }
 
 @end