Add a new connection option(BBRR) to enable BBR's rate based recovery. Protected by FLAGS_quic_rel…

net/quic/core/congestion_control/bbr_sender.h

Index: net/quic/core/congestion_control/bbr_sender.h
diff --git a/net/quic/core/congestion_control/bbr_sender.h b/net/quic/core/congestion_control/bbr_sender.h
index 31102b7d91f33483880b73bff1009d1cdfb2cf02..8b736867d264caa8d03abdf88a129d64a01d6f73 100644
--- a/net/quic/core/congestion_control/bbr_sender.h
+++ b/net/quic/core/congestion_control/bbr_sender.h
@@ -334,6 +334,9 @@ class QUIC_EXPORT_PRIVATE BbrSender : public SendAlgorithmInterface {
   std::deque<DataDelivered> recently_acked_;
   QuicByteCount bytes_recently_acked_;
 
+  // When true, recovery is rate based rather than congestion window based.
+  bool rate_based_recovery_;
+
   DISALLOW_COPY_AND_ASSIGN(BbrSender);
 };