Add GetSources to VideoRtpReceiver

BUG=webrtc:9770

Change-Id: I16143fce6eb727bbab0f6c621aa5b51bc6d28d6b
Reviewed-on: https://webrtc-review.googlesource.com/101600
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24858}
diff --git a/media/base/mediachannel.h b/media/base/mediachannel.h
index c7d8580..44beb8f 100644
--- a/media/base/mediachannel.h
+++ b/media/base/mediachannel.h
@@ -784,6 +784,8 @@
   virtual void FillBitrateInfo(BandwidthEstimationInfo* bwe_info) = 0;
   // Gets quality stats for the channel.
   virtual bool GetStats(VideoMediaInfo* info) = 0;
+
+  virtual std::vector<webrtc::RtpSource> GetSources(uint32_t ssrc) const = 0;
 };
 
 enum DataMessageType {