Implemented the GetSources() in native code.

webrtc/pc/rtpreceiver.cc

Index: webrtc/pc/rtpreceiver.cc
diff --git a/webrtc/pc/rtpreceiver.cc b/webrtc/pc/rtpreceiver.cc
index 6073b15a9739872fa89fbdb212722d35238e6519..537ebd4b56afd03b4f480ee27207d4fa79132f68 100644
--- a/webrtc/pc/rtpreceiver.cc
+++ b/webrtc/pc/rtpreceiver.cc
@@ -12,7 +12,9 @@
 
 #include "webrtc/api/mediastreamtrackproxy.h"
 #include "webrtc/api/videosourceproxy.h"
+#include "webrtc/base/ptr_util.h"
 #include "webrtc/base/trace_event.h"
+#include "webrtc/modules/rtp_rtcp/include/rtp_receiver.h"
 #include "webrtc/pc/audiotrack.h"
 #include "webrtc/pc/videotrack.h"
 
@@ -97,6 +99,18 @@ void AudioRtpReceiver::Stop() {
   stopped_ = true;
 }
 
+std::vector<std::unique_ptr<RtpContributingSourceInterface>>
+AudioRtpReceiver::GetContributingSources() {
+  std::vector<std::unique_ptr<RtpContributingSourceInterface>>
+      contributing_sources;
+  auto sources = channel_->GetContributingSources(ssrc_);

[the sun, 2017/03/30 11:31:42]

Note, you can add the ssrc entry here and avoid propagating that up the stack -
just pick the csrc with the latest time stamp.

[Taylor Brandstetter, 2017/03/30 22:55:38]

But the AudioRtpReceiver only knows about the SSRC it uses as an identifier for
the receive stream. If the actual SSRC of the receive stream changes (handled in
RtpReceiverImpl::CheckSSRCChanged?), it wouldn't know.

[the sun, 2017/03/31 07:05:42]

Hmm. Thanks for bringing this up. When does it happen in practice?

+  for (auto source : sources) {
+    contributing_sources.push_back(
+        rtc::MakeUnique<RtpContributingSource>(*source));
+  }
+  return contributing_sources;
+}
+
 void AudioRtpReceiver::Reconfigure() {
   RTC_DCHECK(!stopped_);
   if (!channel_) {