Implemented the GetSources() in native code.

webrtc/audio/audio_receive_stream.h

Index: webrtc/audio/audio_receive_stream.h
diff --git a/webrtc/audio/audio_receive_stream.h b/webrtc/audio/audio_receive_stream.h
index adac88387613a43a7f6220567a75c8e881fe313c..01bf728bf934708111049f2347c37cfd4de5d6c5 100644
--- a/webrtc/audio/audio_receive_stream.h
+++ b/webrtc/audio/audio_receive_stream.h
@@ -12,6 +12,7 @@
 #define WEBRTC_AUDIO_AUDIO_RECEIVE_STREAM_H_
 
 #include <memory>
+#include <vector>
 
 #include "webrtc/api/audio/audio_mixer.h"
 #include "webrtc/audio/audio_state.h"
@@ -23,6 +24,7 @@
 namespace webrtc {
 class PacketRouter;
 class RtcEventLog;
+class RtpContributingSource;
 class RtpPacketReceived;
 
 namespace voe {
@@ -49,6 +51,7 @@ class AudioReceiveStream final : public webrtc::AudioReceiveStream,
   int GetOutputLevel() const override;
   void SetSink(std::unique_ptr<AudioSinkInterface> sink) override;
   void SetGain(float gain) override;
+  const std::vector<RtpContributingSource*>& GetContributingSources() override;
 
   // TODO(nisse): Intended to be part of an RtpPacketReceiver interface.
   void OnRtpPacket(const RtpPacketReceived& packet);