Implemented the GetSources() in native code.

webrtc/media/base/mediachannel.h

Index: webrtc/media/base/mediachannel.h
diff --git a/webrtc/media/base/mediachannel.h b/webrtc/media/base/mediachannel.h
index ca02c72fcdeee3092dc37c4529b51b37fa77a4d9..a0087d02c805b9524f35b56eab9cae8790cdfa7d 100644
--- a/webrtc/media/base/mediachannel.h
+++ b/webrtc/media/base/mediachannel.h
@@ -43,6 +43,7 @@ class Timing;
 
 namespace webrtc {
 class AudioSinkInterface;
+class RtpContributingSource;
 class VideoFrame;
 }
 
@@ -1006,6 +1007,9 @@ class VoiceMediaChannel : public MediaChannel {
   virtual void SetRawAudioSink(
       uint32_t ssrc,
       std::unique_ptr<webrtc::AudioSinkInterface> sink) = 0;
+
+  virtual const std::vector<webrtc::RtpContributingSource*>&
+  GetContributingSources(uint32_t ssrc) = 0;
 };
 
 // TODO(deadbeef): Rename to VideoSenderParameters, since they're intended to