Implemented the GetSources() in native code.

webrtc/voice_engine/channel_proxy.h

Index: webrtc/voice_engine/channel_proxy.h
diff --git a/webrtc/voice_engine/channel_proxy.h b/webrtc/voice_engine/channel_proxy.h
index fc894a94223cb1c70ac87aa9be279de098350476..18f629af3216e764cd9f49c5da3a09cc6f61f7d5 100644
--- a/webrtc/voice_engine/channel_proxy.h
+++ b/webrtc/voice_engine/channel_proxy.h
@@ -29,6 +29,7 @@ class PacketRouter;
 class RtcEventLog;
 class RtcpBandwidthObserver;
 class RtcpRttStats;
+class RtpContributingSource;
 class RtpPacketSender;
 class RtpPacketReceived;
 class RtpReceiver;
@@ -120,6 +121,7 @@ class ChannelProxy {
   virtual void OnRecoverableUplinkPacketLossRate(
       float recoverable_packet_loss_rate);
   virtual void RegisterLegacyReceiveCodecs();
+  virtual const std::vector<RtpContributingSource*>& GetContributingSources();
 
  private:
   Channel* channel() const;