Update from https://crrev.com/304418

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 5f2059731672e601776819fb08323e718d53315e..2d0efd769af54f11b16c6e676db88d8e780fd04c 100644
--- a/net/quic/congestion_control/tcp_cubic_sender.cc
+++ b/net/quic/congestion_control/tcp_cubic_sender.cc
@@ -31,6 +31,7 @@ TcpCubicSender::TcpCubicSender(
     const QuicClock* clock,
     const RttStats* rtt_stats,
     bool reno,
+    QuicPacketCount initial_tcp_congestion_window,
     QuicPacketCount max_tcp_congestion_window,
     QuicConnectionStats* stats)
     : hybrid_slow_start_(clock),
@@ -43,7 +44,7 @@ TcpCubicSender::TcpCubicSender(
       largest_sent_sequence_number_(0),
       largest_acked_sequence_number_(0),
       largest_sent_at_last_cutback_(0),
-      congestion_window_(kDefaultInitialWindow),
+      congestion_window_(initial_tcp_congestion_window),
       previous_congestion_window_(0),
       slowstart_threshold_(max_tcp_congestion_window),
       previous_slowstart_threshold_(0),
@@ -55,20 +56,19 @@ TcpCubicSender::~TcpCubicSender() {
   UMA_HISTOGRAM_COUNTS("Net.QuicSession.FinalTcpCwnd", congestion_window_);
 }
 
-void TcpCubicSender::SetFromConfig(const QuicConfig& config, bool is_server) {
+void TcpCubicSender::SetFromConfig(const QuicConfig& config,
+                                   bool is_server,
+                                   bool using_pacing) {
   if (is_server) {
     if (config.HasReceivedConnectionOptions() &&
         ContainsQuicTag(config.ReceivedConnectionOptions(), kIW10)) {
-      // Initial window experiment.  Ignore the initial congestion
-      // window suggested by the client and use the default ICWND of
-      // 10 instead.
-      congestion_window_ = kDefaultInitialWindow;
-    } else if (config.HasReceivedInitialCongestionWindow()) {
-      // Set the initial window size.
-      congestion_window_ = max(kMinimumCongestionWindow,
-          min(kMaxInitialWindow,
-              static_cast<QuicPacketCount>(
-                  config.ReceivedInitialCongestionWindow())));
+      // Initial window experiment.
+      congestion_window_ = 10;
+    }
+    if (using_pacing) {
+      // Disable the ack train mode in hystart when pacing is enabled, since it
+      // may be falsely triggered.
+      hybrid_slow_start_.set_ack_train_detection(false);
     }
   }
 }