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

net/quic/quic_sent_entropy_manager.cc

 // Copyright 2013 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/quic_sent_entropy_manager.h"
 
 #include "base/logging.h"
 #include "net/base/linked_hash_map.h"
 
 using std::make_pair;
 using std::max;
 using std::min;
 
 namespace net {
 
 QuicSentEntropyManager::QuicSentEntropyManager()
     : packets_entropy_hash_(0) {}
 
 QuicSentEntropyManager::~QuicSentEntropyManager() {}
 
 void QuicSentEntropyManager::RecordPacketEntropyHash(
     QuicPacketSequenceNumber sequence_number,
     QuicPacketEntropyHash entropy_hash) {
-  // TODO(satyamshekhar): Check this logic again when/if we enable packet
-  // reordering.
+  if (!packets_entropy_.empty()) {
+    // Ensure packets always are recorded in order.
+    // Every packet's entropy is recorded, even if it's not sent, so there
+    // are not sequence number gaps.
+    DCHECK_LT(packets_entropy_.back().first, sequence_number);
+  }
   packets_entropy_hash_ ^= entropy_hash;
   packets_entropy_.insert(
       make_pair(sequence_number,
                 make_pair(entropy_hash, packets_entropy_hash_)));
   DVLOG(2) << "setting cumulative sent entropy hash to: "
            << static_cast<int>(packets_entropy_hash_)
            << " updated with sequence number " << sequence_number
            << " entropy hash: " << static_cast<int>(entropy_hash);
 }
 
@@ skipping 42 matching lines @@
   while (it->first < sequence_number) {
     packets_entropy_.erase(it);
     it = packets_entropy_.begin();
     DCHECK(it != packets_entropy_.end());
   }
   DVLOG(2) << "Cleared entropy before: "
            << packets_entropy_.begin()->first;
 }
 
 }  // namespace net