Default enable --gfe2_reloadable_flag_quic_run_default_signature_callback_once.

Regression test is being submitted at cl/385903795.

PiperOrigin-RevId: 386022830

quic/core/quic_flags_list.h

diff --git a/quic/core/quic_flags_list.h b/quic/core/quic_flags_list.h
index 64e87ff..910f610 100644
--- a/quic/core/quic_flags_list.h
+++ b/quic/core/quic_flags_list.h
@@ -24,7 +24,7 @@
 // If true, QuicIdleNetworkDetector::SetAlarm will become a noop if dectection has been stopped by QuicIdleNetworkDetector::StopDetection.
 QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_idle_network_detector_no_alarm_after_stopped, false)
 // If true, TlsServerHandshaker::DefaultProofSourceHandle::DefaultSignatureCallback will run at most once.
-QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_run_default_signature_callback_once, false)
+QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_run_default_signature_callback_once, true)
 // If true, abort async QPACK header decompression in QuicSpdyStream::Reset() and in QuicSpdyStream::OnStreamReset().
 QUIC_FLAG(FLAGS_quic_reloadable_flag_quic_abort_qpack_on_stream_reset, true)
 // If true, ack frequency frame can be sent from server to client.