Enable QUIC_VERSION_47 in Quartc.

QUIC_VERSION_47 enables IETF-style variable-length connection ids, including the
ability to specify a zero-length connection id in the client->server direction.
This allows Quartc to shave 8 bytes off every client->server packet, since only
one QUIC connection exists on an ICE port used for Quartc, regardless of
perspective.

gfe-relnote: n/a (Quartc only)
PiperOrigin-RevId: 240653151
Change-Id: Iaa3b3eb7be3606d03deb6dc78ee254b942693137

quic/quartc/quartc_factory.cc

diff --git a/quic/quartc/quartc_factory.cc b/quic/quartc/quartc_factory.cc
index 51066f4..9517e0b 100644
--- a/quic/quartc/quartc_factory.cc
+++ b/quic/quartc/quartc_factory.cc
@@ -57,9 +57,12 @@
   // Fix b/110259444.
   SetQuicReloadableFlag(quic_fix_spurious_ack_alarm, true);
 
-  // Enable version 46+ to enable SendMessage API and 'quic bit' per draft 17.
+  // Enable version 46 to enable SendMessage API and 'quic bit' per draft 17.
   SetQuicReloadableFlag(quic_enable_version_46, true);
 
+  // Enable version 47 to enable variable-length connection ids.
+  SetQuicReloadableFlag(quic_enable_version_47, true);
+
   // Fix for inconsistent reporting of crypto handshake.
   SetQuicReloadableFlag(quic_fix_has_pending_crypto_data, true);