Re-add the QUIC DCHECK to ensure packets are always sent in the order of

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/rtt_stats.h"
@@ skipping 152 matching lines @@
                                   QuicByteCount /*bytes_in_flight*/,
                                   QuicPacketSequenceNumber sequence_number,
                                   QuicByteCount bytes,
                                   HasRetransmittableData is_retransmittable) {
   // Only update bytes_in_flight_ for data packets.
   if (is_retransmittable != HAS_RETRANSMITTABLE_DATA) {
     return false;
   }
 
   prr_out_ += bytes;
-  if (largest_sent_sequence_number_ < sequence_number) {
-    // TODO(rch): Ensure that packets are really sent in order.
-    // DCHECK_LT(largest_sent_sequence_number_, sequence_number);
-    largest_sent_sequence_number_ = sequence_number;
-  }
+  DCHECK_LT(largest_sent_sequence_number_, sequence_number);
+  largest_sent_sequence_number_ = sequence_number;
   hybrid_slow_start_.OnPacketSent(sequence_number);
   return true;
 }
 
 QuicTime::Delta TcpCubicSender::TimeUntilSend(
     QuicTime /* now */,
     QuicByteCount bytes_in_flight,
     HasRetransmittableData has_retransmittable_data) const {
   if (has_retransmittable_data == NO_RETRANSMITTABLE_DATA) {
     // For TCP we can always send an ACK immediately.
@@ skipping 169 matching lines @@
     return QuicTime::Delta::Zero();
   }
   return QuicTime::Delta::Infinite();
 }
 
 CongestionControlType TcpCubicSender::GetCongestionControlType() const {
   return reno_ ? kReno : kCubic;
 }
 
 }  // namespace net