Fix quic::GetPacketHeaderSize when QuicVersionHasLongHeaderLengths is false

gfe-relnote: Bugfix in quic::GetPacketHeaderSize protected by gfe2_reloadable_flag_quic_fix_get_packet_header_size
PiperOrigin-RevId: 256043642
Change-Id: I1f943400e15a684b5cb84f2a11cbb294859f669e
diff --git a/quic/quartc/quartc_factory.cc b/quic/quartc/quartc_factory.cc
index 53eb34b..e609d65 100644
--- a/quic/quartc/quartc_factory.cc
+++ b/quic/quartc/quartc_factory.cc
@@ -82,6 +82,9 @@
   SetQuicReloadableFlag(quic_bbr_less_probe_rtt, true);   // Enable BBR6,7,8.
   SetQuicReloadableFlag(quic_unified_iw_options, true);   // Enable IWXX opts.
   SetQuicReloadableFlag(quic_bbr_flexible_app_limited, true);  // Enable BBR9.
+
+  // Fix GetPacketHeaderSize
+  SetQuicReloadableFlag(quic_fix_get_packet_header_size, true);
 }
 
 QuicConfig CreateQuicConfig(const QuartcSessionConfig& quartc_session_config) {