Implemented the GetSources() in native code.

webrtc/api/test/mock_rtpreceiver.h

 /*
  *  Copyright 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #ifndef WEBRTC_API_TEST_MOCK_RTPRECEIVER_H_
 #define WEBRTC_API_TEST_MOCK_RTPRECEIVER_H_
 
 #include <string>
+#include <vector>
 
 #include "webrtc/api/rtpreceiverinterface.h"
 #include "webrtc/test/gmock.h"
 
 namespace webrtc {
 
 class MockRtpReceiver : public rtc::RefCountedObject<RtpReceiverInterface> {
  public:
   MOCK_METHOD1(SetTrack, void(MediaStreamTrackInterface*));
   MOCK_CONST_METHOD0(track, rtc::scoped_refptr<MediaStreamTrackInterface>());
   MOCK_CONST_METHOD0(media_type, cricket::MediaType());
   MOCK_CONST_METHOD0(id, std::string());
   MOCK_CONST_METHOD0(GetParameters, RtpParameters());
   MOCK_METHOD1(SetParameters, bool(const RtpParameters&));
   MOCK_METHOD1(SetObserver, void(RtpReceiverObserverInterface*));
+  MOCK_CONST_METHOD0(GetSources, std::vector<RtpSource>());
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_TEST_MOCK_RTPRECEIVER_H_