Added a callback function OnAddTrack to PeerConnectionObserver

webrtc/api/peerconnection_unittest.cc

Index: webrtc/api/peerconnection_unittest.cc
diff --git a/webrtc/api/peerconnection_unittest.cc b/webrtc/api/peerconnection_unittest.cc
index 18718c31a674fa744139ba7810e7fc3adc8459b2..0a874f5b4cfdf6219972bf98a846361741d34e20 100644
--- a/webrtc/api/peerconnection_unittest.cc
+++ b/webrtc/api/peerconnection_unittest.cc
@@ -416,6 +416,10 @@ class PeerConnectionTestClient : public webrtc::PeerConnectionObserver,
     }
   }
 
+  void OnAddTrack(
+      rtc::scoped_refptr<webrtc::RtpReceiverInterface> receiver,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) override {}
+
   void SetVideoConstraints(const webrtc::FakeConstraints& video_constraint) {
     video_constraints_ = video_constraint;
   }