Added a callback function OnAddTrack to PeerConnectionObserver

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

Index: webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
index b4a873847de5bda226a1c72d23a48b7eff97f1bb..ae66e55a8bdfc265dc3ee59d6f8ea5a4b4ab937c 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm
@@ -200,6 +200,11 @@ void PeerConnectionDelegateAdapter::OnIceCandidatesRemoved(
                     didRemoveIceCandidates:ice_candidates];
 }
 
+void PeerConnectionDelegateAdapter::OnAddTrack(
+        rtc::scoped_refptr<RtpReceiverInterface> receiver,
+        std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) {
+}
+
 }  // namespace webrtc