Implemented the GetSources() in native code.

webrtc/media/engine/fakewebrtccall.h

Index: webrtc/media/engine/fakewebrtccall.h
diff --git a/webrtc/media/engine/fakewebrtccall.h b/webrtc/media/engine/fakewebrtccall.h
index 6b2542245e7e88eb22cf84d5da1831d344192043..403666193622b57f0cebdc33398c94f064701055 100644
--- a/webrtc/media/engine/fakewebrtccall.h
+++ b/webrtc/media/engine/fakewebrtccall.h
@@ -33,6 +33,10 @@
 #include "webrtc/video_receive_stream.h"
 #include "webrtc/video_send_stream.h"
 
+namespace webrtc {
+class RtpContributingSources;
+}  // namespace webrtc
+
 namespace cricket {
 class FakeAudioSendStream final : public webrtc::AudioSendStream {
  public:
@@ -97,6 +101,9 @@ class FakeAudioReceiveStream final : public webrtc::AudioReceiveStream {
   int GetOutputLevel() const override { return 0; }
   void SetSink(std::unique_ptr<webrtc::AudioSinkInterface> sink) override;
   void SetGain(float gain) override;
+  std::vector<webrtc::RtpContributingSource> GetContributingSources() override {
+    return std::vector<webrtc::RtpContributingSource>();
+  }
 
   int id_ = -1;
   webrtc::AudioReceiveStream::Config config_;