In QUIC BBRv2, support NetworkParams.max_initial_congestion_window when bootstrapping cwnd. This makes BBRv2 behave the same w.r.t NetworkParams.max_initial_congestion_window. Protected by FLAGS_quic_reloadable_flag_quic_bbr2_support_max_bootstrap_cwnd. PiperOrigin-RevId: 336959204 Change-Id: I888f067c6c0dcb3eaaa1b9191a83dd7ec16344c0