Update from https://crrev.com/304418

net/quic/congestion_control/hybrid_slow_start.cc

Index: net/quic/congestion_control/hybrid_slow_start.cc
diff --git a/net/quic/congestion_control/hybrid_slow_start.cc b/net/quic/congestion_control/hybrid_slow_start.cc
index 4b1e8426ce14ad1c05e318cc287246d557e59049..e3789b45f8a76668658d9397ffae7a91dab4859b 100644
--- a/net/quic/congestion_control/hybrid_slow_start.cc
+++ b/net/quic/congestion_control/hybrid_slow_start.cc
@@ -23,6 +23,7 @@ const int64 kHybridStartDelayMaxThresholdUs = 16000;
 
 HybridSlowStart::HybridSlowStart(const QuicClock* clock)
     : clock_(clock),
+      ack_train_detection_(true),
       started_(false),
       hystart_found_(NOT_FOUND),
       last_sent_sequence_number_(0),
@@ -84,12 +85,8 @@ bool HybridSlowStart::ShouldExitSlowStart(QuicTime::Delta latest_rtt,
   // more than the capacity.
   // This first trigger will not come into play until we hit roughly 9.6 Mbps
   // with delayed acks (or 4.8Mbps without delayed acks)
-  // TODO(ianswett): QUIC always uses delayed acks, even at the beginning, so
-  // this should likely be at least 4ms.
-  // TODO(pwestin): we need to make sure our pacing don't trigger this detector.
-  // TODO(ianswett): Pacing or other cases could be handled by checking the send
-  // time of the first acked packet in a receive round.
-  if (current_time.Subtract(last_close_ack_pair_time_).ToMicroseconds() <=
+  if (ack_train_detection_ &&
+      current_time.Subtract(last_close_ack_pair_time_).ToMicroseconds() <=
           kHybridStartDelayMinThresholdUs) {
     last_close_ack_pair_time_ = current_time;
     if (current_time.Subtract(round_start_).ToMicroseconds() >=