Fix a QUIC DCHECK flakiness caused by not checking if a packet is

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 5c2b139a6e3fb13c5c0d2bfd675f88b67f59e774..b7b28c48de020d88856cca7b2a1dd6a872a75706 100644
--- a/net/quic/quic_sent_packet_manager.cc
+++ b/net/quic/quic_sent_packet_manager.cc
@@ -610,11 +610,13 @@ void QuicSentPacketManager::RetransmitCryptoPackets() {
   for (UnackedPacketMap::iterator it = unacked_packets_.begin();
        it != unacked_packets_.end(); ++it) {
     QuicPacketSequenceNumber sequence_number = it->first;
-    DCHECK(ContainsKey(packet_history_map_, sequence_number));
     const RetransmittableFrames* frames = it->second.retransmittable_frames;
-    if (frames == NULL || frames->HasCryptoHandshake() != IS_HANDSHAKE) {
+    // Only retransmit frames which are pending, and therefore have been sent.
+    if (!it->second.pending || frames == NULL ||
+        frames->HasCryptoHandshake() != IS_HANDSHAKE) {
       continue;
     }
+    DCHECK(ContainsKey(packet_history_map_, sequence_number));
     packet_retransmitted = true;
     MarkForRetransmission(sequence_number, TLP_RETRANSMISSION);
     // Abandon all the crypto retransmissions now so they're not lost later.
@@ -630,7 +632,8 @@ void QuicSentPacketManager::RetransmitOldestPacket() {
        it != unacked_packets_.end(); ++it) {
     QuicPacketSequenceNumber sequence_number = it->first;
     const RetransmittableFrames* frames = it->second.retransmittable_frames;
-    if (frames == NULL) {
+    // Only retransmit frames which are pending, and therefore have been sent.
+    if (!it->second.pending || frames == NULL) {
       continue;
     }
     DCHECK_NE(IS_HANDSHAKE, frames->HasCryptoHandshake());
@@ -723,7 +726,7 @@ void QuicSentPacketManager::MaybeRetransmitOnAckFrame(
     DCHECK(IsAwaitingPacket(received_info, sequence_number));
     DCHECK(ContainsKey(packet_history_map_, sequence_number));
     const TransmissionInfo& transmission_info = it->second;
-    const SendAlgorithmInterface::SentPacket* sent_packet =
+    SendAlgorithmInterface::SentPacket* sent_packet =
         packet_history_map_[sequence_number];
 
     // Consider it multiple nacks when there is a gap between the missing packet
@@ -732,7 +735,7 @@ void QuicSentPacketManager::MaybeRetransmitOnAckFrame(
     // TODO(ianswett): This relies heavily on sequential reception of packets,
     // and makes an assumption that the congestion control uses TCP style nacks.
     size_t min_nacks = received_info.largest_observed - sequence_number;
-    packet_history_map_[sequence_number]->Nack(min_nacks);
+    sent_packet->Nack(min_nacks);
 
     size_t num_nacks_needed = kNumberOfNacksBeforeRetransmission;
     // Check for early retransmit(RFC5827) when the last packet gets acked and