Update from https://crrev.com/311076

net/quic/congestion_control/tcp_loss_algorithm.cc

Index: net/quic/congestion_control/tcp_loss_algorithm.cc
diff --git a/net/quic/congestion_control/tcp_loss_algorithm.cc b/net/quic/congestion_control/tcp_loss_algorithm.cc
index 9366105e82229c04fb1408fe211c1a374a145c94..f657cc225fe5ac217f57bcdfa6b51f4006728e15 100644
--- a/net/quic/congestion_control/tcp_loss_algorithm.cc
+++ b/net/quic/congestion_control/tcp_loss_algorithm.cc
@@ -10,10 +10,17 @@
 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()) { }
@@ -30,8 +37,10 @@ SequenceNumberSet TCPLossAlgorithm::DetectLostPackets(
     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;
@@ -49,6 +58,16 @@ SequenceNumberSet TCPLossAlgorithm::DetectLostPackets(
       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.
@@ -56,12 +75,12 @@ SequenceNumberSet TCPLossAlgorithm::DetectLostPackets(
         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;
       }
     }