Update from https://crrev.com/304418

net/quic/congestion_control/hybrid_slow_start_test.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 "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "net/quic/congestion_control/hybrid_slow_start.h"
 #include "net/quic/test_tools/mock_clock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 36 matching lines @@
   }
   EXPECT_TRUE(slow_start_->IsEndOfRound(sequence_number++));
 }
 
 // TODO(ianswett): Add tests which more realistically invoke the methods,
 // simulating how actual acks arrive and packets are sent.
 TEST_F(HybridSlowStartTest, AckTrain) {
   // At a typical RTT 60 ms, assuming that the inter arrival timestamp is 1 ms,
   // we expect to be able to send a burst of 30 packet before we trigger the
   // ack train detection.
-  const int kMaxLoopCount = 5;
-  QuicPacketSequenceNumber sequence_number = 2;
-  QuicPacketSequenceNumber end_sequence_number = 2;
-  for (int burst = 0; burst < kMaxLoopCount; ++burst) {
+  // Run this test for both enabled and disabled ack train detection.
+  for (int i = 0; i < 2; ++i) {
+    const bool ack_train_detection = (i == 1);
+    slow_start_->set_ack_train_detection(ack_train_detection);
+
+    const int kMaxLoopCount = 5;
+    QuicPacketSequenceNumber sequence_number = 2;
+    QuicPacketSequenceNumber end_sequence_number = 2;
+    for (int burst = 0; burst < kMaxLoopCount; ++burst) {
+      slow_start_->StartReceiveRound(end_sequence_number);
+      do {
+        clock_.AdvanceTime(one_ms_);
+        EXPECT_FALSE(slow_start_->ShouldExitSlowStart(rtt_, rtt_, 100));
+      } while (!slow_start_->IsEndOfRound(sequence_number++));
+      end_sequence_number *= 2;  // Exponential growth.
+    }
     slow_start_->StartReceiveRound(end_sequence_number);
-    do {
+
+    for (int n = 0;
+         n < 29 && !slow_start_->IsEndOfRound(sequence_number++); ++n) {
       clock_.AdvanceTime(one_ms_);
       EXPECT_FALSE(slow_start_->ShouldExitSlowStart(rtt_, rtt_, 100));
-    }  while (!slow_start_->IsEndOfRound(sequence_number++));
-    end_sequence_number *= 2;  // Exponential growth.
+    }
+    clock_.AdvanceTime(one_ms_);
+    EXPECT_EQ(ack_train_detection,
+              slow_start_->ShouldExitSlowStart(rtt_, rtt_, 100));
   }
-  slow_start_->StartReceiveRound(end_sequence_number);
-
-  for (int n = 0;
-       n < 29 && !slow_start_->IsEndOfRound(sequence_number++); ++n) {
-    clock_.AdvanceTime(one_ms_);
-    EXPECT_FALSE(slow_start_->ShouldExitSlowStart(rtt_, rtt_, 100));
-  }
-  clock_.AdvanceTime(one_ms_);
-  EXPECT_TRUE(slow_start_->ShouldExitSlowStart(rtt_, rtt_, 100));
 }
 
 TEST_F(HybridSlowStartTest, Delay) {
   // We expect to detect the increase at +1/16 of the RTT; hence at a typical
   // RTT of 60ms the detection will happen at 63.75 ms.
   const int kHybridStartMinSamples = 8;  // Number of acks required to trigger.
 
   QuicPacketSequenceNumber end_sequence_number = 1;
   slow_start_->StartReceiveRound(end_sequence_number++);
 
   // Will not trigger since our lowest RTT in our burst is the same as the long
   // term RTT provided.
   for (int n = 0; n < kHybridStartMinSamples; ++n) {
     EXPECT_FALSE(slow_start_->ShouldExitSlowStart(
         rtt_.Add(QuicTime::Delta::FromMilliseconds(n)), rtt_, 100));
   }
   slow_start_->StartReceiveRound(end_sequence_number++);
   for (int n = 1; n < kHybridStartMinSamples; ++n) {
     EXPECT_FALSE(slow_start_->ShouldExitSlowStart(
         rtt_.Add(QuicTime::Delta::FromMilliseconds(n + 5)), rtt_, 100));
   }
   // Expect to trigger since all packets in this burst was above the long term
   // RTT provided.
   EXPECT_TRUE(slow_start_->ShouldExitSlowStart(
       rtt_.Add(QuicTime::Delta::FromMilliseconds(5)), rtt_, 100));
 }
 
 }  // namespace test
 }  // namespace net