Implemented the GetSources() in native code.

webrtc/api/rtpparameters.h

Index: webrtc/api/rtpparameters.h
diff --git a/webrtc/api/rtpparameters.h b/webrtc/api/rtpparameters.h
index a428804d54b91ebd43571a916728afdbf29d27f6..95cf04731b9e6ee0dd85ecca0ca964d1171beba0 100644
--- a/webrtc/api/rtpparameters.h
+++ b/webrtc/api/rtpparameters.h
@@ -67,7 +67,7 @@ enum class DtxStatus {
   ENABLED,
 };
 
-enum class DegradationPreference {

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

This is changed because it accidentally conflict with a name in the video
encoder part.

[Taylor Brandstetter, 2017/03/31 22:10:50]

It looks like the two enums are referring to the same thing. Could they just be
unified? May be better in a standalone CL.

+enum class RtpDegradationPreference {
   MAINTAIN_FRAMERATE,
   MAINTAIN_RESOLUTION,
   BALANCED,
@@ -441,8 +441,8 @@ struct RtpParameters {
   std::vector<RtpEncodingParameters> encodings;
 
   // TODO(deadbeef): Not implemented.
-  DegradationPreference degradation_preference =
-      DegradationPreference::BALANCED;
+  RtpDegradationPreference degradation_preference =
+      RtpDegradationPreference::BALANCED;
 
   bool operator==(const RtpParameters& o) const {
     return mid == o.mid && codecs == o.codecs &&