Implemented the GetSources() in native code.

webrtc/voice_engine/channel.h

Index: webrtc/voice_engine/channel.h
diff --git a/webrtc/voice_engine/channel.h b/webrtc/voice_engine/channel.h
index 24a7d6bfa0dc91457758d1f1aaeb0dc464ac7521..8ff151ef6fdaf67c1be4262d057c134468408400 100644
--- a/webrtc/voice_engine/channel.h
+++ b/webrtc/voice_engine/channel.h
@@ -27,6 +27,7 @@
 #include "webrtc/modules/audio_processing/rms_level.h"
 #include "webrtc/modules/rtp_rtcp/include/remote_ntp_time_estimator.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_header_parser.h"
+#include "webrtc/modules/rtp_rtcp/include/rtp_receiver.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp.h"
 #include "webrtc/voice_engine/audio_level.h"
 #include "webrtc/voice_engine/file_player.h"
@@ -388,6 +389,10 @@ class Channel
 
   void OnRecoverableUplinkPacketLossRate(float recoverable_packet_loss_rate);
 
+  const std::vector<RtpContributingSource>& GetContributingSources() {
+    return rtp_receiver_->GetContributingSources();
+  }
+
  private:
   void OnUplinkPacketLossRate(float packet_loss_rate);