Add a flag to enable QUIC's BBR congestion control by default instead of

net/quic/quic_flags.cc

Index: net/quic/quic_flags.cc
diff --git a/net/quic/quic_flags.cc b/net/quic/quic_flags.cc
index affa27129c3beebf701e343c2ef58eb89f4d5aa7..73e492eb420fe944f40846411d1b7e0353d58b6a 100644
--- a/net/quic/quic_flags.cc
+++ b/net/quic/quic_flags.cc
@@ -38,3 +38,6 @@ bool FLAGS_enable_quic_fec = false;
 
 // If true, a QUIC connection with too many unfinished streams will be closed.
 bool FLAGS_close_quic_connection_unfinished_streams = false;
+
+// When true, defaults to BBR congestion control instead of Cubic.
+bool FLAGS_quic_use_bbr_congestion_control = false;