Replace the map in QuicAckNotifier with a counter.

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 3f54f9b225b213b60cdd038dd311947f4a09c22c..34bb9726f38266d40b64007883d9245618620d04 100644
--- a/net/quic/quic_sent_packet_manager.cc
+++ b/net/quic/quic_sent_packet_manager.cc
@@ -576,10 +576,10 @@ bool QuicSentPacketManager::OnPacketSent(
                    << "pending_retransmissions_.  sequence_number: "
                    << original_sequence_number;
     }
-    // A notifier may be waiting to hear about ACKs for the original sequence
-    // number. Inform them that the sequence number has changed.
-    ack_notifier_manager_.UpdateSequenceNumber(original_sequence_number,
-                                               sequence_number);
+    // Inform the ack notifier of retransmissions so it can calculate the
+    // retransmit rate.
+    ack_notifier_manager_.OnPacketRetransmitted(original_sequence_number,
+                                                sequence_number, bytes);
   }
 
   if (pending_timer_transmission_count_ > 0) {