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

net/quic/chromium/quic_stream_factory.h

Index: net/quic/chromium/quic_stream_factory.h
diff --git a/net/quic/chromium/quic_stream_factory.h b/net/quic/chromium/quic_stream_factory.h
index b0fef1fb7c8e3591cbfe692c9bfe437e574b8e62..7aaae69f294eb18534e1f075bbc1f7ade99d6e47 100644
--- a/net/quic/chromium/quic_stream_factory.h
+++ b/net/quic/chromium/quic_stream_factory.h
@@ -196,6 +196,7 @@ class NET_EXPORT_PRIVATE QuicStreamFactory
       bool allow_server_migration,
       bool force_hol_blocking,
       bool race_cert_verification,
+      bool quic_do_not_fragment,
       const QuicTagVector& connection_options,
       bool enable_token_binding);
   ~QuicStreamFactory() override;
@@ -623,6 +624,9 @@ class NET_EXPORT_PRIVATE QuicStreamFactory
   // Set if cert verification is to be raced with host resolution.
   bool race_cert_verification_;
 
+  // If set, configure QUIC sockets to not fragment packets.
+  bool quic_do_not_fragment_;
+
   // Each profile will (probably) have a unique port_seed_ value.  This value
   // is used to help seed a pseudo-random number generator (PortSuggester) so
   // that we consistently (within this profile) suggest the same ephemeral