Added OnAddTrack to Objective C SDK.

Exposed native OnAddTrack event in Objective C SDK
peer connection delegate via
peerConnection:didAddReceiver:streams:

Bug: webrtc:6112
Change-Id: Iccf33ab7844c9a774a6b54e49de011d100998f03
Reviewed-on: https://webrtc-review.googlesource.com/56980
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22253}
diff --git a/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h b/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
index 8b69804..92c0d2f 100644
--- a/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
+++ b/sdk/objc/Framework/Headers/WebRTC/RTCPeerConnection.h
@@ -120,6 +120,12 @@
 - (void)peerConnection:(RTCPeerConnection *)peerConnection
     didStartReceivingOnTransceiver:(RTCRtpTransceiver *)transceiver;
 
+/** Called when a receiver and its track are created. */
+@optional
+- (void)peerConnection:(RTCPeerConnection *)peerConnection
+        didAddReceiver:(RTCRtpReceiver *)rtpReceiver
+               streams:(NSArray<RTCMediaStream *> *)mediaStreams;
+
 @end
 
 RTC_EXPORT