Update from https://crrev.com/311076

net/quic/congestion_control/tcp_cubic_sender.h

 // 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.
 //
 // TCP cubic send side congestion algorithm, emulates the behavior of
 // TCP cubic.
 
 #ifndef NET_QUIC_CONGESTION_CONTROL_TCP_CUBIC_SENDER_H_
 #define NET_QUIC_CONGESTION_CONTROL_TCP_CUBIC_SENDER_H_
 
@@ skipping 99 matching lines @@
 
   // Track the largest packet that has been acked.
   QuicPacketSequenceNumber largest_acked_sequence_number_;
 
   // Track the largest sequence number outstanding when a CWND cutback occurs.
   QuicPacketSequenceNumber largest_sent_at_last_cutback_;
 
   // Congestion window in packets.
   QuicPacketCount congestion_window_;
 
-  // Congestion window before the last loss event or RTO.
+  // Congestion window before the last RTO.
+  // Must be 0 before or after RTO recovery.
   QuicPacketCount previous_congestion_window_;
 
   // Slow start congestion window in packets, aka ssthresh.
   QuicPacketCount slowstart_threshold_;
 
   // Slow start threshold before the last loss event or RTO.
   QuicPacketCount previous_slowstart_threshold_;
 
   // Whether the last loss event caused us to exit slowstart.
   // Used for stats collection of slowstart_packets_lost
   bool last_cutback_exited_slowstart_;
 
   // Maximum number of outstanding packets for tcp.
   QuicPacketCount max_tcp_congestion_window_;
 
   const QuicClock* clock_;
 
   DISALLOW_COPY_AND_ASSIGN(TcpCubicSender);
 };
 
 }  // namespace net
 
 #endif  // NET_QUIC_CONGESTION_CONTROL_TCP_CUBIC_SENDER_H_