Add histogram stats for AV sync stream offset:

webrtc/system_wrappers/source/rtp_to_ntp.cc

Index: webrtc/system_wrappers/source/rtp_to_ntp.cc
diff --git a/webrtc/system_wrappers/source/rtp_to_ntp.cc b/webrtc/system_wrappers/source/rtp_to_ntp.cc
index 0aceb0625f81a925b96cf55514181adff920778e..706d861b55c82b0457516131c87b2412e87c16ca 100644
--- a/webrtc/system_wrappers/source/rtp_to_ntp.cc
+++ b/webrtc/system_wrappers/source/rtp_to_ntp.cc
@@ -95,7 +95,9 @@ bool UpdateRtcpList(uint32_t ntp_secs,
 bool RtpToNtpMs(int64_t rtp_timestamp,
                 const RtcpList& rtcp,
                 int64_t* rtp_timestamp_in_ms) {
-  assert(rtcp.size() == 2);
+  if (rtcp.size() != 2)
+    return false;
+
   int64_t rtcp_ntp_ms_new = Clock::NtpToMs(rtcp.front().ntp_secs,
                                            rtcp.front().ntp_frac);
   int64_t rtcp_ntp_ms_old = Clock::NtpToMs(rtcp.back().ntp_secs,