Implemented the GetSources() in native code.

webrtc/pc/channel.h

Index: webrtc/pc/channel.h
diff --git a/webrtc/pc/channel.h b/webrtc/pc/channel.h
index 6ff0556c9798525ffbe3d29b739dc55085c8681c..8b8af086564c19568311f2ddf0674c75cb73e754 100644
--- a/webrtc/pc/channel.h
+++ b/webrtc/pc/channel.h
@@ -491,6 +491,9 @@ class VoiceChannel : public BaseChannel {
   // Get statistics about the current media session.
   bool GetStats(VoiceMediaInfo* stats);
 
+  std::vector<webrtc::RtpContributingSource*> GetContributingSources(
+      uint32_t ssrc);
+
   // Monitoring functions
   sigslot::signal2<VoiceChannel*, const std::vector<ConnectionInfo>&>
       SignalConnectionMonitor;
@@ -532,7 +535,6 @@ class VoiceChannel : public BaseChannel {
   void HandleEarlyMediaTimeout();
   bool InsertDtmf_w(uint32_t ssrc, int event, int duration);
   bool SetOutputVolume_w(uint32_t ssrc, double volume);
-  bool GetStats_w(VoiceMediaInfo* stats);
 
   void OnMessage(rtc::Message* pmsg) override;
   void GetSrtpCryptoSuites_n(std::vector<int>* crypto_suites) const override;