Implemented the GetSources() in native code.

webrtc/call/audio_receive_stream.h

Index: webrtc/call/audio_receive_stream.h
diff --git a/webrtc/call/audio_receive_stream.h b/webrtc/call/audio_receive_stream.h
index 3959da1369eeb1cd11dc9eecad6b536d93b789b8..f72b5da7e99abff93f983ba724fe0ab35e2ee304 100644
--- a/webrtc/call/audio_receive_stream.h
+++ b/webrtc/call/audio_receive_stream.h
@@ -26,6 +26,7 @@
 
 namespace webrtc {
 class AudioSinkInterface;
+class RtpContributingSource;
 
 // WORK IN PROGRESS
 // This class is under development and is not yet intended for for use outside
@@ -133,6 +134,9 @@ class AudioReceiveStream {
   // is potentially forwarded to any attached AudioSinkInterface implementation.
   virtual void SetGain(float gain) = 0;
 
+  virtual const std::vector<RtpContributingSource*>&
+  GetContributingSources() = 0;
+
  protected:
   virtual ~AudioReceiveStream() {}
 };