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 86b5e65304619a099b2d0cb22ba07f03700e7a35..565f88ce42c69b122bd9d438ff89ad263aeb0a56 100644
--- a/webrtc/voice_engine/channel_proxy.h
+++ b/webrtc/voice_engine/channel_proxy.h
@@ -12,6 +12,7 @@
 #define WEBRTC_VOICE_ENGINE_CHANNEL_PROXY_H_
 
 #include "webrtc/api/audio/audio_mixer.h"
+#include "webrtc/api/rtpreceiverinterface.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/base/race_checker.h"
 #include "webrtc/base/thread_checker.h"
@@ -121,6 +122,7 @@ class ChannelProxy {
   virtual void OnRecoverableUplinkPacketLossRate(
       float recoverable_packet_loss_rate);
   virtual void RegisterLegacyReceiveCodecs();
+  virtual std::vector<webrtc::RtpSource> GetSources() const;
 
  private:
   Channel* channel() const;