gfe-relnote: Default-initialize QUIC BBRv2 loss event threshold for exiting STARTUP from a flag. Protected by --gfe2_reloadable_flag_quic_default_to_bbr_v2.

PiperOrigin-RevId: 264298542
Change-Id: I304ab19e4820dec51d3f8ef53762a393f6b175fd
diff --git a/quic/core/congestion_control/bbr2_misc.h b/quic/core/congestion_control/bbr2_misc.h
index 0dee438..4ad351e 100644
--- a/quic/core/congestion_control/bbr2_misc.h
+++ b/quic/core/congestion_control/bbr2_misc.h
@@ -87,7 +87,8 @@
   QuicRoundTripCount startup_full_bw_rounds = 3;
 
   // The minimum number of loss marking events to exit STARTUP.
-  int64_t startup_full_loss_count = 8;
+  int64_t startup_full_loss_count =
+      GetQuicFlag(FLAGS_quic_bbr2_default_startup_full_loss_count);
 
   /*
    * DRAIN parameters.