Add a LOG(DFATAL) to a case where packets are acked before being sent.

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 6b2b9deae10e6f800c379a1b20bfd4f359cc041d..12ddf29f7441943375c31b2e587cef2a59da0fa9 100644
--- a/net/quic/quic_sent_packet_manager.cc
+++ b/net/quic/quic_sent_packet_manager.cc
@@ -720,8 +720,10 @@ bool QuicSentPacketManager::MaybeUpdateRTT(
   // sequence numbers will include the ACK aggregation delay.
   const TransmissionInfo& transmission_info =
       unacked_packets_.GetTransmissionInfo(ack_frame.largest_observed);
-  // Don't update the RTT if it hasn't been sent.
+  // Ensure the packet has a valid sent time.
   if (transmission_info.sent_time == QuicTime::Zero()) {
+    LOG(DFATAL) << "Acked packet has zero sent time, largest_observed:"

[ramant (doing other things), 2014/09/15 19:56:44]

rch, ian: should we make this DLOG(DFATAL) in chromium? Or this should never
happen and we want to crash the browser if it happens.

[Ryan Hamilton, 2014/09/15 21:44:54]

I don't have strong feelings about LOG(DFATAL) vs DLOG(DFATAL). LOG(DFATAL) ==
LOG(ERROR) in non-debug builds so we won't crash in either case.

[ramant (doing other things), 2014/09/15 21:47:50]

Sounds great. 

+                << ack_frame.largest_observed;
     return false;
   }