Add statistics gathering for packet loss.

Adds a class used to classify whether packet loss events are a single packet or multiple packets as well as how many packets have been lost. Also exposes a new function in the RtpRtcp interface to retrieve these statistics.

BUG=

Review URL: https://codereview.webrtc.org/1198853004

Cr-Commit-Position: refs/heads/master@{#9568}

webrtc/modules/rtp_rtcp/interface/rtp_rtcp_defines.h

@@ -350,5 +350,18 @@ class NullRtpAudioFeedback : public RtpAudioFeedback {
                             const uint8_t volume) override {}
 };
 
+// Statistics about packet loss for a single directional connection. All values
+// are totals since the connection initiated.
+struct RtpPacketLossStats {
+  // The number of packets lost in events where no adjacent packets were also
+  // lost.
+  uint64_t single_packet_loss_count;
+  // The number of events in which more than one adjacent packet was lost.
+  uint64_t multiple_packet_loss_event_count;
+  // The number of packets lost in events where more than one adjacent packet
+  // was lost.
+  uint64_t multiple_packet_loss_packet_count;
+};
+
 }  // namespace webrtc
 #endif // WEBRTC_MODULES_RTP_RTCP_INTERFACE_RTP_RTCP_DEFINES_H_