Implemented the GetSources() in native code.

webrtc/media/base/fakemediaengine.h

Index: webrtc/media/base/fakemediaengine.h
diff --git a/webrtc/media/base/fakemediaengine.h b/webrtc/media/base/fakemediaengine.h
index fc59fa7d7d889c20b755271391df9bdc79019a5a..25698f7eb7e979fc8d7c7bae8075531667afa313 100644
--- a/webrtc/media/base/fakemediaengine.h
+++ b/webrtc/media/base/fakemediaengine.h
@@ -418,6 +418,11 @@ class FakeVoiceMediaChannel : public RtpHelper<VoiceMediaChannel> {
 
   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) {
@@ -499,6 +504,7 @@ class FakeVoiceMediaChannel : public RtpHelper<VoiceMediaChannel> {
   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.