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.h

@@ -430,6 +430,14 @@ class RtpRtcp : public Module {
         StreamDataCounters* rtp_counters,
         StreamDataCounters* rtx_counters) const = 0;
 
+    /*
+     *  Get packet loss statistics for the RTP stream.
+     */
+    virtual void GetRtpPacketLossStats(
+        bool outgoing,
+        uint32_t ssrc,
+        struct RtpPacketLossStats* loss_stats) const = 0;
+
     /*
     *   Get received RTCP sender info
     *