Fix a bug in QuicSentPacketManager::ClearPreviousRetransmissions where

net/quic/quic_sent_packet_manager.cc

Index: net/quic/quic_sent_packet_manager.cc
diff --git a/net/quic/quic_sent_packet_manager.cc b/net/quic/quic_sent_packet_manager.cc
index c7a5663f0eea7cddbe7b3acfd63cb5102c5f8339..65c0254434c4e93e0e6eb2a9a726c2e68a4a2a31 100644
--- a/net/quic/quic_sent_packet_manager.cc
+++ b/net/quic/quic_sent_packet_manager.cc
@@ -255,6 +255,13 @@ void QuicSentPacketManager::ClearPreviousRetransmissions(size_t num_to_clear) {
     SequenceNumberSet* previous_transmissions =
         it->second.previous_transmissions;
     if (previous_transmissions == NULL) {
+      if (it->second.retransmittable_frames == NULL) {
+        // This is a current transmission, but a previous transmission has
+        // been acked, so it's safe to remove.
+        it = MarkPacketHandled(sequence_number, NOT_RECEIVED_BY_PEER);
+        --num_to_clear;
+        continue;
+      }
       break;
     }
     QuicPacketSequenceNumber newest_transmission =