Add field to BoostIcwHeader proto message: h3_force_enable_quic_bbr_exit_startup_on_loss

This lets AFEs enable BBRv1's exit-startup-on-loss behavior on established QUIC connections via the "Sec-Google-GFE-Boost-ICW" response header.

Protected by FLAGS_quic_reloadable_flag_quic_bbr_exit_startup_on_loss_network_param.

PiperOrigin-RevId: 882252755

quiche/common/quiche_feature_flags_list.h

diff --git a/quiche/common/quiche_feature_flags_list.h b/quiche/common/quiche_feature_flags_list.h
index 6c9ad32..c640cdc 100755
--- a/quiche/common/quiche_feature_flags_list.h
+++ b/quiche/common/quiche_feature_flags_list.h
@@ -16,6 +16,7 @@
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_bbr2_extra_acked_window, false, true, "When true, the BBR4 copt sets the extra_acked window to 20 RTTs and BBR5 sets it to 40 RTTs.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_bbr_always_exit_startup_on_loss, false, false, "Lets BBRv1 exit STARTUP if a large number of packets was lost even if there is no non-app-limited samples. Similar to applying connection option B1AL to all connection.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_bbr_exit_startup_on_loss, true, true, "Enables negotiation of a connection option (B1AL) that lets BBRv1 exit STARTUP if a large number of packets was lost even if there is no non-app-limited sample.")
+QUICHE_FLAG(bool, quiche_reloadable_flag_quic_bbr_exit_startup_on_loss_network_param, false, false, "Enables adjustment of BBRv1's exit-startup-on-loss behavior on established connections via NetworkParams::enable_bbr_exit_startup_on_loss.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_block_until_settings_received_copt, true, true, "If enabled and a BSUS connection is received, blocks server connections until SETTINGS frame is received.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_clear_body_manager_along_with_sequencer, false, false, "If true, QuicSpdyStream::StopReading always clears BodyManager along with the SequenceBuffer.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_client_check_blockage_before_on_can_write, false, false, "If true, quic clients will only call OnCanWrite() upon write events if the writer is unblocked.")