Fix a crash bug in QuicUnackedPacketMap when truncated acks arrived.

net/quic/quic_unacked_packet_map_test.cc

Index: net/quic/quic_unacked_packet_map_test.cc
diff --git a/net/quic/quic_unacked_packet_map_test.cc b/net/quic/quic_unacked_packet_map_test.cc
index 16d1df2bcde22953a6e00146448f5e3d22a35223..0a7bb2b490ea43801f7997e3fe187050becbbf5f 100644
--- a/net/quic/quic_unacked_packet_map_test.cc
+++ b/net/quic/quic_unacked_packet_map_test.cc
@@ -234,6 +234,19 @@ TEST_F(QuicUnackedPacketMapTest, RetransmitThreeTimes) {
   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