Update from https://crrev.com/311076

net/quic/congestion_control/tcp_loss_algorithm.cc

 // Copyright 2014 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/congestion_control/tcp_loss_algorithm.h"
 
 #include "net/quic/congestion_control/rtt_stats.h"
 #include "net/quic/quic_protocol.h"
 
 namespace net {
 
 namespace {
+
+// The minimum delay before a packet will be considered lost,
+// regardless of SRTT.  Half of the minimum TLP, since the loss algorithm only
+// triggers when a nack has been receieved for the packet.
+static const size_t kMinLossDelayMs = 5;
+
 // How many RTTs the algorithm waits before determining a packet is lost due
 // to early retransmission.
 static const double kEarlyRetransmitLossDelayMultiplier = 1.25;
-}
+
+}  // namespace
 
 TCPLossAlgorithm::TCPLossAlgorithm()
     : loss_detection_timeout_(QuicTime::Zero()) { }
 
 LossDetectionType TCPLossAlgorithm::GetLossDetectionType() const {
   return kNack;
 }
 
 // Uses nack counts to decide when packets are lost.
 SequenceNumberSet TCPLossAlgorithm::DetectLostPackets(
     const QuicUnackedPacketMap& unacked_packets,
     const QuicTime& time,
     QuicPacketSequenceNumber largest_observed,
     const RttStats& rtt_stats) {
   SequenceNumberSet lost_packets;
   loss_detection_timeout_ = QuicTime::Zero();
-  QuicTime::Delta loss_delay =
-      rtt_stats.smoothed_rtt().Multiply(kEarlyRetransmitLossDelayMultiplier);
+  QuicTime::Delta early_retransmit_delay = QuicTime::Delta::Max(
+      QuicTime::Delta::FromMilliseconds(kMinLossDelayMs),
+      rtt_stats.smoothed_rtt().Multiply(kEarlyRetransmitLossDelayMultiplier));
+
   QuicPacketSequenceNumber sequence_number = unacked_packets.GetLeastUnacked();
   for (QuicUnackedPacketMap::const_iterator it = unacked_packets.begin();
        it != unacked_packets.end() && sequence_number <= largest_observed;
        ++it, ++sequence_number) {
     if (!it->in_flight) {
       continue;
     }
 
     LOG_IF(DFATAL, it->nack_count == 0 && it->sent_time.IsInitialized())
         << "All packets less than largest observed should have been nacked."
         << "sequence_number:" << sequence_number
         << " largest_observed:" << largest_observed;
     if (it->nack_count >= kNumberOfNacksBeforeRetransmission) {
       lost_packets.insert(sequence_number);
       continue;
     }
 
+    // Immediately lose the packet if it's been an srtt between the sent time
+    // of it and the largest observed.  This speeds recovery from timer based
+    // retransmissions, such as TLP and RTO, when there may be fewer than
+    // kNumberOfNacksBeforeRetransmission nacks.
+    if (it->sent_time.Add(rtt_stats.smoothed_rtt()) <
+        unacked_packets.GetTransmissionInfo(largest_observed).sent_time) {
+      lost_packets.insert(sequence_number);
+      continue;
+    }
+
     // Only early retransmit(RFC5827) when the last packet gets acked and
     // there are retransmittable packets in flight.
     // This also implements a timer-protected variant of FACK.
     if (it->retransmittable_frames &&
         unacked_packets.largest_sent_packet() == largest_observed) {
       // Early retransmit marks the packet as lost once 1.25RTTs have passed
       // since the packet was sent and otherwise sets an alarm.
-      if (time >= it->sent_time.Add(loss_delay)) {
+      if (time >= it->sent_time.Add(early_retransmit_delay)) {
         lost_packets.insert(sequence_number);
       } else {
         // Set the timeout for the earliest retransmittable packet where early
         // retransmit applies.
-        loss_detection_timeout_ = it->sent_time.Add(loss_delay);
+        loss_detection_timeout_ = it->sent_time.Add(early_retransmit_delay);
         break;
       }
     }
   }
 
   return lost_packets;
 }
 
 QuicTime TCPLossAlgorithm::GetLossTimeout() const {
   return loss_detection_timeout_;
 }
 
 }  // namespace net