Use a field trial to decide if QUIC should attempt to prevent packet fragmentation.

net/quic/chromium/quic_stream_factory_test.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/chromium/quic_stream_factory.h"
 
 #include <ostream>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 344 matching lines @@
         prefer_aes_, max_number_of_lossy_connections_, packet_loss_threshold_,
         max_disabled_reasons_, threshold_timeouts_with_open_streams_,
         threshold_public_resets_post_handshake_, receive_buffer_size_,
         delay_tcp_race_, /*max_server_configs_stored_in_properties*/ 0,
         close_sessions_on_ip_change_,
         disable_quic_on_timeout_with_open_streams_,
         idle_connection_timeout_seconds_,
         packet_reader_yield_after_duration_milliseconds_,
         migrate_sessions_on_network_change_, migrate_sessions_early_,
         allow_server_migration_, force_hol_blocking_, race_cert_verification_,
-        QuicTagVector(),
+        /*do_not_fragment*/ true, QuicTagVector(),
         /*enable_token_binding*/ false));
     factory_->set_require_confirmation(false);
     EXPECT_FALSE(factory_->has_quic_server_info_factory());
     factory_->set_quic_server_info_factory(new MockQuicServerInfoFactory());
     EXPECT_TRUE(factory_->has_quic_server_info_factory());
   }
 
   void InitializeConnectionMigrationTest(
       NetworkChangeNotifier::NetworkList connected_networks) {
     scoped_mock_network_change_notifier_.reset(
@@ skipping 4709 matching lines @@
   // Clear all cached states.
   factory_->ClearCachedStatesInCryptoConfig(
       base::Callback<bool(const GURL&)>());
   EXPECT_TRUE(test_cases[0].state->certs().empty());
   EXPECT_TRUE(test_cases[1].state->certs().empty());
   EXPECT_TRUE(test_cases[2].state->certs().empty());
 }
 
 }  // namespace test
 }  // namespace net