Default to BBR congestion control for QUIC. Protected by

net/quic/core/quic_connection.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/core/quic_connection.h"
 
 #include <string.h>
 #include <sys/types.h>
 
 #include <algorithm>
@@ skipping 235 matching lines @@
       packet_generator_(connection_id_,
                         &framer_,
                         random_generator_,
                         helper->GetBufferAllocator(),
                         this),
       idle_network_timeout_(QuicTime::Delta::Infinite()),
       handshake_timeout_(QuicTime::Delta::Infinite()),
       time_of_last_received_packet_(clock_->ApproximateNow()),
       time_of_last_sent_new_packet_(clock_->ApproximateNow()),
       last_send_for_timeout_(clock_->ApproximateNow()),
-      sent_packet_manager_(perspective, clock_, &stats_, kCubicBytes, kNack),
+      sent_packet_manager_(
+          perspective,
+          clock_,
+          &stats_,
+          FLAGS_quic_reloadable_flag_quic_default_to_bbr ? kBBR : kCubicBytes,
+          kNack),
       version_negotiation_state_(START_NEGOTIATION),
       perspective_(perspective),
       connected_(true),
       can_truncate_connection_ids_(true),
       mtu_discovery_target_(0),
       mtu_probe_count_(0),
       packets_between_mtu_probes_(kPacketsBetweenMtuProbesBase),
       next_mtu_probe_at_(kPacketsBetweenMtuProbesBase),
       largest_received_packet_size_(0),
       goaway_sent_(false),
@@ skipping 2163 matching lines @@
     sent_packet_manager_.OnApplicationLimited();
   }
 }
 
 void QuicConnection::SetStreamNotifier(
     StreamNotifierInterface* stream_notifier) {
   sent_packet_manager_.SetStreamNotifier(stream_notifier);
 }
 
 }  // namespace net