Implemented the GetSources() in native code.

webrtc/pc/rtpreceiver.cc

 /*
  *  Copyright 2015 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 79 matching lines @@
     return;
   }
   if (channel_) {
     // Allow that SetOutputVolume fail. This is the normal case when the
     // underlying media channel has already been deleted.
     channel_->SetOutputVolume(ssrc_, 0);
   }
   stopped_ = true;
 }
 
+std::vector<RtpSource> AudioRtpReceiver::GetSources() const {
+  return channel_->GetSources(ssrc_);
+}
+
 void AudioRtpReceiver::Reconfigure() {
   RTC_DCHECK(!stopped_);
   if (!channel_) {
     LOG(LS_ERROR) << "AudioRtpReceiver::Reconfigure: No audio channel exists.";
     return;
   }
   if (!channel_->SetOutputVolume(ssrc_,
                                  track_->enabled() ? cached_volume_ : 0)) {
     RTC_NOTREACHED();
   }
@@ skipping 119 matching lines @@
 }
 
 void VideoRtpReceiver::OnFirstPacketReceived(cricket::BaseChannel* channel) {
   if (observer_) {
     observer_->OnFirstPacketReceived(media_type());
   }
   received_first_packet_ = true;
 }
 
 }  // namespace webrtc