Fix a bug in QuicUnackedPacketMap where an in flight packet could be

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 a07de8b3a8b890f5f535de7bff141487aa91fd7c..185d995b1d2c0bbd43cd067591fa0fd8c3b3a1a3 100644
--- a/net/quic/quic_unacked_packet_map.cc
+++ b/net/quic/quic_unacked_packet_map.cc
@@ -80,8 +80,13 @@ void QuicUnackedPacketMap::OnRetransmittedPacket(
       *(++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);
+    TransmissionInfo* old_transmission_info =
+        FindOrNull(unacked_packets_, old_transmission);
+    // Don't remove old packets if they're still in flight.
+    if (old_transmission_info == NULL || !old_transmission_info->in_flight) {
+      transmission_info->all_transmissions->erase(old_transmission);
+      unacked_packets_.erase(old_transmission);
+    }
   }
   unacked_packets_[new_sequence_number] =
       TransmissionInfo(frames,