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

net/quic/chromium/quic_stream_factory_test.cc

Index: net/quic/chromium/quic_stream_factory_test.cc
diff --git a/net/quic/chromium/quic_stream_factory_test.cc b/net/quic/chromium/quic_stream_factory_test.cc
index fa60f63a0f41d96a7a7232ae955703c6ef709311..4377d057db0d78f9e1567266d3c5dddc77259ea8 100644
--- a/net/quic/chromium/quic_stream_factory_test.cc
+++ b/net/quic/chromium/quic_stream_factory_test.cc
@@ -362,7 +362,7 @@ class QuicStreamFactoryTestBase {
         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());