Fix a crash bug in QuicUnackedPacketMap when truncated acks arrived.

net/quic/quic_unacked_packet_map_test.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/quic_unacked_packet_map.h"
 
 #include "net/quic/test_tools/quic_test_utils.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace net {
@@ skipping 216 matching lines @@
   unacked_packets_.RemoveRetransmittability(6);
   unacked_packets_.OnRetransmittedPacket(5, 7, LOSS_RETRANSMISSION);
   unacked_packets_.SetSent(7, now_, kDefaultLength, true);
 
   QuicPacketSequenceNumber unacked4[] = { 3, 5, 7 };
   VerifyUnackedPackets(unacked4, arraysize(unacked4));
   QuicPacketSequenceNumber pending4[] = { 3, 5, 7 };
   VerifyInFlightPackets(pending4, arraysize(pending4));
   QuicPacketSequenceNumber retransmittable4[] = { 7 };
   VerifyRetransmittablePackets(retransmittable4, arraysize(retransmittable4));
+
+  // Remove the older two transmissions from in flight.
+  unacked_packets_.RemoveFromInFlight(3);
+  unacked_packets_.RemoveFromInFlight(5);
+  QuicPacketSequenceNumber pending5[] = { 7 };
+  VerifyInFlightPackets(pending5, arraysize(pending5));
+
+  // Now test ClearPreviousTransmissions.
+  unacked_packets_.ClearPreviousRetransmissions(5);
+  QuicPacketSequenceNumber unacked5[] = { 7 };
+  VerifyUnackedPackets(unacked5, arraysize(unacked5));
+  QuicPacketSequenceNumber retransmittable5[] = { 7 };
+  VerifyRetransmittablePackets(retransmittable5, arraysize(retransmittable5));
 }
 
 }  // namespace
 }  // namespace test
 }  // namespace net