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..fd1a89eb0a00ad046ad3b9831615a70f900e51a6 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"
@@ -49,6 +50,7 @@ class AudioReceiveStream final : public webrtc::AudioReceiveStream,
   int GetOutputLevel() const override;
   void SetSink(std::unique_ptr<AudioSinkInterface> sink) override;
   void SetGain(float gain) override;
+  std::vector<webrtc::RtpContributingSource> GetContributingSources() override;
 
   // TODO(nisse): Intended to be part of an RtpPacketReceiver interface.
   void OnRtpPacket(const RtpPacketReceived& packet);