Implemented the GetSources() in native code.

webrtc/media/base/fakemediaengine.h

 /*
  *  Copyright (c) 2004 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 400 matching lines @@
   }
   bool GetOutputVolume(uint32_t ssrc, double* volume) {
     if (output_scalings_.find(ssrc) == output_scalings_.end())
       return false;
     *volume = output_scalings_[ssrc];
     return true;
   }
 
   virtual bool GetStats(VoiceMediaInfo* info) { return false; }
 
+  virtual const std::vector<webrtc::RtpContributingSource*>&
+  GetContributingSources(uint32_t ssrc) {
+    return contributing_sources_;
+  }
+
   virtual void SetRawAudioSink(
       uint32_t ssrc,
       std::unique_ptr<webrtc::AudioSinkInterface> sink) {
     sink_ = std::move(sink);
   }
 
  private:
   class VoiceChannelAudioSink : public AudioSource::Sink {
    public:
     explicit VoiceChannelAudioSink(AudioSource* source) : source_(source) {
@@ skipping 61 matching lines @@
 
   FakeVoiceEngine* engine_;
   std::vector<AudioCodec> recv_codecs_;
   std::vector<AudioCodec> send_codecs_;
   std::map<uint32_t, double> output_scalings_;
   std::vector<DtmfInfo> dtmf_info_queue_;
   AudioOptions options_;
   std::map<uint32_t, VoiceChannelAudioSink*> local_sinks_;
   std::unique_ptr<webrtc::AudioSinkInterface> sink_;
   int max_bps_;
+  std::vector<webrtc::RtpContributingSource*> contributing_sources_;
 };
 
 // A helper function to compare the FakeVoiceMediaChannel::DtmfInfo.
 inline bool CompareDtmfInfo(const FakeVoiceMediaChannel::DtmfInfo& info,
                             uint32_t ssrc,
                             int event_code,
                             int duration) {
   return (info.duration == duration && info.event_code == event_code &&
           info.ssrc == ssrc);
 }
@@ skipping 450 matching lines @@
   virtual const std::vector<DataCodec>& data_codecs() { return data_codecs_; }
 
  private:
   std::vector<FakeDataMediaChannel*> channels_;
   std::vector<DataCodec> data_codecs_;
 };
 
 }  // namespace cricket
 
 #endif  // WEBRTC_MEDIA_BASE_FAKEMEDIAENGINE_H_