Ensure the UnackedPacketMap only keeps one previous transmission less

net/quic/quic_unacked_packet_map.cc

Index: net/quic/quic_unacked_packet_map.cc
diff --git a/net/quic/quic_unacked_packet_map.cc b/net/quic/quic_unacked_packet_map.cc
index 563b53dfe92d1da8dd141d729b36def31d342aa4..a07de8b3a8b890f5f535de7bff141487aa91fd7c 100644
--- a/net/quic/quic_unacked_packet_map.cc
+++ b/net/quic/quic_unacked_packet_map.cc
@@ -74,6 +74,15 @@ void QuicUnackedPacketMap::OnRetransmittedPacket(
   // We keep the old packet in the unacked packet list until it, or one of
   // the retransmissions of it are acked.
   transmission_info->retransmittable_frames = NULL;
+  // Only keep one transmission older than largest observed, because only the
+  // most recent is expected to possibly be a spurious retransmission.
+  if (transmission_info->all_transmissions->size() > 1 &&
+      *(++transmission_info->all_transmissions->begin()) < largest_observed_) {
+    QuicPacketSequenceNumber old_transmission =
+        *transmission_info->all_transmissions->begin();
+    transmission_info->all_transmissions->erase(old_transmission);
+    unacked_packets_.erase(old_transmission);
+  }
   unacked_packets_[new_sequence_number] =
       TransmissionInfo(frames,
                        new_sequence_number,