Allow QUIC's BBR TCP sender to pace based on min rtt and add an explicit

net/quic/congestion_control/tcp_cubic_sender.cc

Index: net/quic/congestion_control/tcp_cubic_sender.cc
diff --git a/net/quic/congestion_control/tcp_cubic_sender.cc b/net/quic/congestion_control/tcp_cubic_sender.cc
index 0aae5fdea714c109ed5fbcf3c3aff448fa03d74d..0a0b6ae38bde0cd8344f5d0194f7639c893fd435 100644
--- a/net/quic/congestion_control/tcp_cubic_sender.cc
+++ b/net/quic/congestion_control/tcp_cubic_sender.cc
@@ -204,6 +204,13 @@ QuicByteCount TcpCubicSender::SendWindow() const {
   return min(receive_window_, GetCongestionWindow());
 }
 
+QuicBandwidth TcpCubicSender::PacingRate() const {
+  // We pace at twice the rate of the underlying sender's bandwidth estimate
+  // during slow start and 1.25x during congestion avoidance to ensure pacing
+  // doesn't prevent us from filling the window.
+  return BandwidthEstimate().Scale(InSlowStart() ? 2 : 1.25);
+}
+
 QuicBandwidth TcpCubicSender::BandwidthEstimate() const {
   return QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(),
                                               rtt_stats_->SmoothedRtt());
@@ -217,9 +224,8 @@ QuicTime::Delta TcpCubicSender::RetransmissionDelay() const {
   if (!rtt_stats_->HasUpdates()) {
     return QuicTime::Delta::Zero();
   }
-  return QuicTime::Delta::FromMicroseconds(
-      rtt_stats_->SmoothedRtt().ToMicroseconds() +
-      4 * rtt_stats_->mean_deviation().ToMicroseconds());
+  return rtt_stats_->SmoothedRtt().Add(
+      rtt_stats_->mean_deviation().Multiply(4));
 }
 
 QuicByteCount TcpCubicSender::GetCongestionWindow() const {