Added a callback function OnAddTrack to PeerConnectionObserver

webrtc/api/android/jni/peerconnection_jni.cc

Index: webrtc/api/android/jni/peerconnection_jni.cc
diff --git a/webrtc/api/android/jni/peerconnection_jni.cc b/webrtc/api/android/jni/peerconnection_jni.cc
index 7012fc355a3f24257abf2326d918ed41e173ab9c..d4f1092a6d505144d4cdd7ae999e9db2d27c7945 100644
--- a/webrtc/api/android/jni/peerconnection_jni.cc
+++ b/webrtc/api/android/jni/peerconnection_jni.cc
@@ -374,6 +374,10 @@ class PCOJava : public PeerConnectionObserver {
     CHECK_EXCEPTION(jni()) << "error during CallVoidMethod";
   }
 
+  void OnAddTrack(
+      rtc::scoped_refptr<RtpReceiverInterface> receiver,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) override {}
+
   void SetConstraints(ConstraintsWrapper* constraints) {
     RTC_CHECK(!constraints_.get()) << "constraints already set!";
     constraints_.reset(constraints);