Add histogram stats for AV sync stream offset:

webrtc/modules/rtp_rtcp/source/remote_ntp_time_estimator.cc

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 33 matching lines @@
   // Update extrapolator with the new arrival time.
   // The extrapolator assumes the TimeInMilliseconds time.
   int64_t receiver_arrival_time_ms = clock_->TimeInMilliseconds();
   int64_t sender_send_time_ms = Clock::NtpToMs(ntp_secs, ntp_frac);
   int64_t sender_arrival_time_90k = (sender_send_time_ms + rtt / 2) * 90;
   ts_extrapolator_->Update(receiver_arrival_time_ms, sender_arrival_time_90k);
   return true;
 }
 
 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;
   }
   uint32_t timestamp = sender_capture_ntp_ms * 90;
   int64_t receiver_capture_ms =
       ts_extrapolator_->ExtrapolateLocalTime(timestamp);
   int64_t ntp_offset =
       clock_->CurrentNtpInMilliseconds() - clock_->TimeInMilliseconds();
   int64_t receiver_capture_ntp_ms = receiver_capture_ms + ntp_offset;
   int64_t now_ms = clock_->TimeInMilliseconds();
   if (now_ms - last_timing_log_ms_ > kTimingLogIntervalMs) {
     LOG(LS_INFO) << "RTP timestamp: " << rtp_timestamp
                  << " in NTP clock: " << sender_capture_ntp_ms
                  << " estimated time in receiver clock: " << receiver_capture_ms
                  << " converted to NTP clock: " << receiver_capture_ntp_ms;
     last_timing_log_ms_ = now_ms;
   }
   return receiver_capture_ntp_ms;
 }
 
 }  // namespace webrtc