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

PiperOrigin-RevId: 262618561
Change-Id: I2f50bb14997eb985beb6f1f3f1ab71a585af595b

quic/core/congestion_control/bbr2_misc.h

diff --git a/quic/core/congestion_control/bbr2_misc.h b/quic/core/congestion_control/bbr2_misc.h
index 303e299..0dee438 100644
--- a/quic/core/congestion_control/bbr2_misc.h
+++ b/quic/core/congestion_control/bbr2_misc.h
@@ -147,7 +147,7 @@
   float inflight_hi_headroom = 0.15;
 
   // Estimate startup/bw probing has gone too far if loss rate exceeds this.
-  float loss_threshold = 0.02;
+  float loss_threshold = GetQuicFlag(FLAGS_quic_bbr2_default_loss_threshold);
 
   Limits<QuicByteCount> cwnd_limits;
 };