Create the Java Wrapper of RtpReceiverObserverInterface.

webrtc/api/rtpreceiver.cc

Index: webrtc/api/rtpreceiver.cc
diff --git a/webrtc/api/rtpreceiver.cc b/webrtc/api/rtpreceiver.cc
index 02e1650cb9ca642cdda9890ad21a9d4de499c60b..80a0256a02b8c6c802ec5bd73da0ce98a80f0879 100644
--- a/webrtc/api/rtpreceiver.cc
+++ b/webrtc/api/rtpreceiver.cc
@@ -114,7 +114,7 @@ void AudioRtpReceiver::Reconfigure() {
 void AudioRtpReceiver::SetObserver(RtpReceiverObserverInterface* observer) {
   observer_ = observer;
   // Deliver any notifications the observer may have missed by being set late.
-  if (received_first_packet_) {
+  if (received_first_packet_ && observer_) {
     observer_->OnFirstPacketReceived(media_type());
   }
 }
@@ -212,7 +212,7 @@ void VideoRtpReceiver::Stop() {
 void VideoRtpReceiver::SetObserver(RtpReceiverObserverInterface* observer) {
   observer_ = observer;
   // Deliver any notifications the observer may have missed by being set late.
-  if (received_first_packet_) {
+  if (received_first_packet_ && observer_) {
     observer_->OnFirstPacketReceived(media_type());
   }
 }