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

components/network_session_configurator/network_session_configurator_unittest.cc

Index: components/network_session_configurator/network_session_configurator_unittest.cc
diff --git a/components/network_session_configurator/network_session_configurator_unittest.cc b/components/network_session_configurator/network_session_configurator_unittest.cc
index c749fa87033245e44e8e67a860bf20f23b09700b..92650fff319986330bdbf591d38bb159d64dd773 100644
--- a/components/network_session_configurator/network_session_configurator_unittest.cc
+++ b/components/network_session_configurator/network_session_configurator_unittest.cc
@@ -83,6 +83,7 @@ TEST_F(NetworkSessionConfiguratorTest, EnableQuicFromFieldTrialGroup) {
   EXPECT_EQ(net::kQuicYieldAfterDurationMilliseconds,
             params_.quic_packet_reader_yield_after_duration_milliseconds);
   EXPECT_FALSE(params_.quic_race_cert_verification);
+  EXPECT_FALSE(params_.quic_do_not_fragment);
   EXPECT_FALSE(params_.quic_disable_preconnect_if_0rtt);
   EXPECT_FALSE(params_.quic_migrate_sessions_on_network_change);
   EXPECT_FALSE(params_.quic_migrate_sessions_early);
@@ -164,6 +165,17 @@ TEST_F(NetworkSessionConfiguratorTest, QuicRaceCertVerification) {
   EXPECT_TRUE(params_.quic_race_cert_verification);
 }
 
+TEST_F(NetworkSessionConfiguratorTest, QuicDoNotFragment) {
+  std::map<std::string, std::string> field_trial_params;
+  field_trial_params["do_not_fragment"] = "true";
+  variations::AssociateVariationParams("QUIC", "Enabled", field_trial_params);
+  base::FieldTrialList::CreateFieldTrial("QUIC", "Enabled");
+
+  ParseFieldTrials();
+
+  EXPECT_TRUE(params_.quic_do_not_fragment);
+}
+
 TEST_F(NetworkSessionConfiguratorTest, QuicDisablePreConnectIfZeroRtt) {
   std::map<std::string, std::string> field_trial_params;
   field_trial_params["disable_preconnect_if_0rtt"] = "true";