New const method StreamStatistician::GetStats

And a corresponding struct RtpReceiveStats. This is intended
to hold the information exposed via GetStats, which is quite
different from the stats reported to the peer via RTCP.

This is a preparation for moving ReceiveStatistics out of the
individual receive stream objects, and instead have a shared instance
owned by RtpStreamReceiverController or maybe Call.

Bug: webrtc:10679,chromium:677543
Change-Id: Ibb52ee769516ddc51da109b7f2319405693be5d5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/148982
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28943}
diff --git a/call/video_receive_stream.h b/call/video_receive_stream.h
index 4bc9b28..ad702e6 100644
--- a/call/video_receive_stream.h
+++ b/call/video_receive_stream.h
@@ -110,9 +110,8 @@
 
     uint32_t ssrc = 0;
     std::string c_name;
-    StreamDataCounters rtp_stats;
+    RtpReceiveStats rtp_stats;
     RtcpPacketTypeCounter rtcp_packet_type_counts;
-    RtcpStatistics rtcp_stats;
 
     // Timing frame info: all important timestamps for a full lifetime of a
     // single 'timing frame'.