Update from https://crrev.com/311076

net/quic/congestion_control/tcp_cubic_sender.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/congestion_control/tcp_cubic_sender.h"
 
 #include <algorithm>
 
 #include "base/metrics/histogram.h"
 #include "net/quic/congestion_control/prr_sender.h"
@@ skipping 120 matching lines @@
     OnPacketAcked(it->first, it->second.bytes_sent, bytes_in_flight);
   }
 }
 
 void TcpCubicSender::OnPacketAcked(
     QuicPacketSequenceNumber acked_sequence_number,
     QuicByteCount acked_bytes,
     QuicByteCount bytes_in_flight) {
   largest_acked_sequence_number_ = max(acked_sequence_number,
                                        largest_acked_sequence_number_);
+  // As soon as a packet is acked, ensure we're no longer in RTO mode.
+  previous_congestion_window_ = 0;
   if (InRecovery()) {
     // PRR is used when in recovery.
     prr_.OnPacketAcked(acked_bytes);
     return;
   }
   MaybeIncreaseCwnd(acked_sequence_number, bytes_in_flight);
   // TODO(ianswett): Should this even be called when not in slow start?
   hybrid_slow_start_.OnPacketAcked(acked_sequence_number, InSlowStart());
 }
 
@@ skipping 187 matching lines @@
   }
 }
 
 void TcpCubicSender::OnRetransmissionTimeout(bool packets_retransmitted) {
   largest_sent_at_last_cutback_ = 0;
   if (!packets_retransmitted) {
     return;
   }
   cubic_.Reset();
   hybrid_slow_start_.Restart();
+  // Only reduce ssthresh once over multiple retransmissions.
+  if (previous_congestion_window_ != 0) {
+    return;
+  }
   previous_slowstart_threshold_ = slowstart_threshold_;
   slowstart_threshold_ = congestion_window_ / 2;
   previous_congestion_window_ = congestion_window_;
   congestion_window_ = kMinimumCongestionWindow;
 }
 
 void TcpCubicSender::RevertRetransmissionTimeout() {
   if (previous_congestion_window_ == 0) {
     LOG(DFATAL) << "No previous congestion window to revert to.";
     return;
   }
   congestion_window_ = previous_congestion_window_;
   slowstart_threshold_ = previous_slowstart_threshold_;
   previous_congestion_window_ = 0;
 }
 
 CongestionControlType TcpCubicSender::GetCongestionControlType() const {
   return reno_ ? kReno : kCubic;
 }
 
 }  // namespace net