Add histogram stats for AV sync stream offset:

webrtc/modules/rtp_rtcp/source/remote_ntp_time_estimator.cc

Index: webrtc/modules/rtp_rtcp/source/remote_ntp_time_estimator.cc
diff --git a/webrtc/modules/rtp_rtcp/source/remote_ntp_time_estimator.cc b/webrtc/modules/rtp_rtcp/source/remote_ntp_time_estimator.cc
index ccc15ec4173b92bbcbe1baf0fb8f5e0a2a97c9b3..69a8f2391f46dc7c57516e5957fef0cc368b5428 100644
--- a/webrtc/modules/rtp_rtcp/source/remote_ntp_time_estimator.cc
+++ b/webrtc/modules/rtp_rtcp/source/remote_ntp_time_estimator.cc
@@ -51,10 +51,6 @@ bool RemoteNtpTimeEstimator::UpdateRtcpTimestamp(int64_t rtt,
 }
 
 int64_t RemoteNtpTimeEstimator::Estimate(uint32_t rtp_timestamp) {
-  if (rtcp_list_.size() < 2) {
-    // We need two RTCP SR reports to calculate NTP.
-    return -1;
-  }
   int64_t sender_capture_ntp_ms = 0;
   if (!RtpToNtpMs(rtp_timestamp, rtcp_list_, &sender_capture_ntp_ms)) {
     return -1;