Implemented the GetSources() in native code.

webrtc/pc/rtpreceiver.h

Index: webrtc/pc/rtpreceiver.h
diff --git a/webrtc/pc/rtpreceiver.h b/webrtc/pc/rtpreceiver.h
index 513f90c7ab3d9a00796868fe32a85bf5069e7d85..9bef83df4827477bb44001f1d091cf8847c3d8e4 100644
--- a/webrtc/pc/rtpreceiver.h
+++ b/webrtc/pc/rtpreceiver.h
@@ -30,6 +30,8 @@
 
 namespace webrtc {
 
+class RtpContributingSource;
+
 // Internal class used by PeerConnection.
 class RtpReceiverInternal : public RtpReceiverInterface {
  public:
@@ -37,6 +39,13 @@ class RtpReceiverInternal : public RtpReceiverInterface {
   // This SSRC is used as an identifier for the receiver between the API layer
   // and the WebRtcVideoEngine2, WebRtcVoiceEngine layer.
   virtual uint32_t ssrc() const = 0;
+
+  // TODO(zhihuang): Remove the default implemenation once the subclasses

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

nit: Spelling of "implementation"

Also, isn't the default implementation needed on RtpReceiverInterface, not
RtpReceiverInternal? And, assuming the only relevant subclass is
"FakeRtpReceiver" in chromium, it may be helpful to mention it by name, as a
reminder to whoever goes to fix the TODO.

[Zhi Huang, 2017/03/31 06:44:05]

Done.

+  // implement this.
+  virtual std::vector<std::unique_ptr<RtpContributingSourceInterface>>
+  GetContributingSources() {
+    return std::vector<std::unique_ptr<RtpContributingSourceInterface>>();
+  }
 };
 
 class AudioRtpReceiver : public ObserverInterface,
@@ -88,6 +97,9 @@ class AudioRtpReceiver : public ObserverInterface,
   // Should call SetChannel(nullptr) before |channel| is destroyed.
   void SetChannel(cricket::VoiceChannel* channel);
 
+  std::vector<std::unique_ptr<RtpContributingSourceInterface>>
+  GetContributingSources() override;
+
  private:
   void Reconfigure();
   void OnFirstPacketReceived(cricket::BaseChannel* channel);