Add a new QuicFlagSaver class for saving/restoring the values of QUIC flags in tests.

net/quic/core/quic_sent_packet_manager_test.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/core/quic_sent_packet_manager.h"
 
 #include <memory>
 #include "base/memory/ptr_util.h"
 #include "base/stl_util.h"
 #include "net/quic/core/quic_flags.h"
@@ skipping 285 matching lines @@
   void NackPackets(QuicPacketNumber lower,
                    QuicPacketNumber higher,
                    QuicAckFrame* frame) {
     if (frame->missing) {
       frame->packets.Add(lower, higher);
     } else {
       frame->packets.Remove(lower, higher);
     }
   }
 
+  QuicFlagSaver flags_;  // Save/restore all QUIC flag values.
   QuicSentPacketManager manager_;
   vector<QuicEncryptedPacket*> packets_;
   MockClock clock_;
   QuicConnectionStats stats_;
   MockSendAlgorithm* send_algorithm_;
   std::unique_ptr<MockNetworkChangeVisitor> network_change_visitor_;
 };
 
 INSTANTIATE_TEST_CASE_P(QuicSentPacketManagerTest,
                         QuicSentPacketManagerTest,
@@ skipping 1391 matching lines @@
   ExpectAck(1);
   EXPECT_CALL(*network_change_visitor_,
               OnPathMtuIncreased(kDefaultLength + 100));
   QuicAckFrame ack_frame = InitAckFrame(1);
   manager_.OnIncomingAck(ack_frame, clock_.Now());
 }
 
 }  // namespace
 }  // namespace test
 }  // namespace net