Implemented the GetSources() in native code.

webrtc/call/audio_receive_stream.h

Index: webrtc/call/audio_receive_stream.h
diff --git a/webrtc/call/audio_receive_stream.h b/webrtc/call/audio_receive_stream.h
index 3959da1369eeb1cd11dc9eecad6b536d93b789b8..e3bdd452473666bc68bebd16116944b84176035f 100644
--- a/webrtc/call/audio_receive_stream.h
+++ b/webrtc/call/audio_receive_stream.h
@@ -18,6 +18,7 @@
 
 #include "webrtc/api/audio_codecs/audio_decoder_factory.h"
 #include "webrtc/api/call/transport.h"
+#include "webrtc/api/rtpreceiverinterface.h"
 #include "webrtc/base/optional.h"
 #include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/common_types.h"
@@ -133,6 +134,8 @@ class AudioReceiveStream {
   // is potentially forwarded to any attached AudioSinkInterface implementation.
   virtual void SetGain(float gain) = 0;
 
+  virtual std::vector<RtpSource> GetSources() const = 0;
+
  protected:
   virtual ~AudioReceiveStream() {}
 };