Implemented the GetSources() in native code.

webrtc/api/test/mock_rtpreceiver.h

Index: webrtc/api/test/mock_rtpreceiver.h
diff --git a/webrtc/api/test/mock_rtpreceiver.h b/webrtc/api/test/mock_rtpreceiver.h
index 25f5c28caa62b3ac2c70121a750b5a0ca9a94cad..a8b92817b222f056009a0c8ce2e12111392c0f63 100644
--- a/webrtc/api/test/mock_rtpreceiver.h
+++ b/webrtc/api/test/mock_rtpreceiver.h
@@ -11,7 +11,9 @@
 #ifndef WEBRTC_API_TEST_MOCK_RTPRECEIVER_H_
 #define WEBRTC_API_TEST_MOCK_RTPRECEIVER_H_
 
+#include <memory>
 #include <string>
+#include <vector>
 
 #include "webrtc/api/rtpreceiverinterface.h"
 #include "webrtc/test/gmock.h"
@@ -27,6 +29,8 @@ class MockRtpReceiver : public rtc::RefCountedObject<RtpReceiverInterface> {
   MOCK_CONST_METHOD0(GetParameters, RtpParameters());
   MOCK_METHOD1(SetParameters, bool(const RtpParameters&));
   MOCK_METHOD1(SetObserver, void(RtpReceiverObserverInterface*));
+  MOCK_METHOD0(GetContributingSources,
+               std::vector<std::unique_ptr<RtpContributingSourceInterface>>());
 };
 
 }  // namespace webrtc