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}

modules/rtp_rtcp/source/receive_statistics_impl.h

@@ -32,6 +32,8 @@ class StreamStatisticianImpl : public StreamStatistician,
                          int max_reordering_threshold);
   ~StreamStatisticianImpl() override;
 
+  RtpReceiveStats GetStats() const override;
+
   // |reset| here and in next method restarts calculation of fraction_lost stat.
   bool GetStatistics(RtcpStatistics* statistics, bool reset) override;
   bool GetActiveStatisticsAndReset(RtcpStatistics* statistics);