Added a callback function OnAddTrack to PeerConnectionObserver

webrtc/api/peerconnectionfactory_unittest.cc

Index: webrtc/api/peerconnectionfactory_unittest.cc
diff --git a/webrtc/api/peerconnectionfactory_unittest.cc b/webrtc/api/peerconnectionfactory_unittest.cc
index 9cd74ceb95fefbb7bdb7a95329269eadf2864189..ae780a839bc96062a19e19d5d72cebea4c910e83 100644
--- a/webrtc/api/peerconnectionfactory_unittest.cc
+++ b/webrtc/api/peerconnectionfactory_unittest.cc
@@ -86,6 +86,9 @@ class NullPeerConnectionObserver : public PeerConnectionObserver {
   virtual void OnIceGatheringChange(
       PeerConnectionInterface::IceGatheringState new_state) {}
   virtual void OnIceCandidate(const webrtc::IceCandidateInterface* candidate) {}
+  virtual void OnAddTrack(
+      rtc::scoped_refptr<webrtc::RtpReceiverInterface> receiver,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) {}
 };
 
 }  // namespace