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

net/quic/core/crypto/crypto_protocol.h

Index: net/quic/core/crypto/crypto_protocol.h
diff --git a/net/quic/core/crypto/crypto_protocol.h b/net/quic/core/crypto/crypto_protocol.h
index d31ef0f10250f13b7d748949779c969946a64c1b..56d8e63844c73d4700a9f6e100f4be136c92b733 100644
--- a/net/quic/core/crypto/crypto_protocol.h
+++ b/net/quic/core/crypto/crypto_protocol.h
@@ -82,6 +82,7 @@ const QuicTag kIFWA = TAG('I', 'F', 'W', 'a');   // Set initial size
 const QuicTag kTBBR = TAG('T', 'B', 'B', 'R');   // Reduced Buffer Bloat TCP
 const QuicTag k1RTT = TAG('1', 'R', 'T', 'T');   // STARTUP in BBR for 1 RTT
 const QuicTag k2RTT = TAG('2', 'R', 'T', 'T');   // STARTUP in BBR for 2 RTTs
+const QuicTag kBBRR = TAG('B', 'B', 'R', 'R');   // Rate-based recovery in BBR
 const QuicTag kRENO = TAG('R', 'E', 'N', 'O');   // Reno Congestion Control
 const QuicTag kTPCC = TAG('P', 'C', 'C', '\0');  // Performance-Oriented
                                                  // Congestion Control