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/include/rtp_rtcp_defines.h

@@ -444,6 +444,20 @@ class StreamDataCountersCallback {
                                    uint32_t ssrc) = 0;
 };
 
+// Information exposed through the GetStats api.
+struct RtpReceiveStats {
+  // |packets_lost| and |jitter| are defined by RFC 3550, and exposed in the
+  // RTCReceivedRtpStreamStats dictionary, see
+  // https://w3c.github.io/webrtc-stats/#receivedrtpstats-dict*
+  int32_t packets_lost = 0;
+  uint32_t jitter = 0;
+
+  // Timestamp and counters exposed in RTCInboundRtpStreamStats, see
+  // https://w3c.github.io/webrtc-stats/#inboundrtpstats-dict*
+  absl::optional<int64_t> last_packet_received_timestamp_ms;
+  RtpPacketCounter packet_counter;
+};
+
 class RtcpAckObserver {
  public:
   // This method is called on received report blocks matching the sender ssrc.