Fix issue with zero rtt reports when using FlexFEC and add perf test.

webrtc/modules/bitrate_controller/send_side_bandwidth_estimation.cc

 /*
  *  Copyright (c) 2012 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 191 matching lines @@
 }
 
 void SendSideBandwidthEstimation::UpdateReceiverBlock(uint8_t fraction_loss,
                                                       int64_t rtt,
                                                       int number_of_packets,
                                                       int64_t now_ms) {
   last_feedback_ms_ = now_ms;
   if (first_report_time_ms_ == -1)
     first_report_time_ms_ = now_ms;
 
-  // Update RTT.
-  last_round_trip_time_ms_ = rtt;
+  // Update RTT if we were able to compute an RTT based on this RTCP.
+  // FlexFEC doesn't send RTCP SR, which means we won't be able to compute RTT.
+  if (rtt > 0)
+    last_round_trip_time_ms_ = rtt;
 
   // Check sequence number diff and weight loss report
   if (number_of_packets > 0) {
     // Calculate number of lost packets.
     const int num_lost_packets_Q8 = fraction_loss * number_of_packets;
     // Accumulate reports.
     lost_packets_since_last_loss_update_Q8_ += num_lost_packets_Q8;
     expected_packets_since_last_loss_update_ += number_of_packets;
 
     // Don't generate a loss rate until it can be based on enough packets.
@@ skipping 187 matching lines @@
       last_fraction_loss_ != last_logged_fraction_loss_ ||
       now_ms - last_rtc_event_log_ms_ > kRtcEventLogPeriodMs) {
     event_log_->LogLossBasedBweUpdate(bitrate_bps, last_fraction_loss_,
                                       expected_packets_since_last_loss_update_);
     last_logged_fraction_loss_ = last_fraction_loss_;
     last_rtc_event_log_ms_ = now_ms;
   }
   current_bitrate_bps_ = bitrate_bps;
 }
 }  // namespace webrtc