Remove an unneeded hash_set from QuicAckNotifierManager.

net/quic/quic_ack_notifier.cc

Index: net/quic/quic_ack_notifier.cc
diff --git a/net/quic/quic_ack_notifier.cc b/net/quic/quic_ack_notifier.cc
index ad76aafa4dfd6299928a33aa9923c5330d51f5dd..6236321bf52bb79d0613bcb3a71e96deb774c182 100644
--- a/net/quic/quic_ack_notifier.cc
+++ b/net/quic/quic_ack_notifier.cc
@@ -36,11 +36,12 @@ void QuicAckNotifier::OnSerializedPacket() {
 bool QuicAckNotifier::OnAck(QuicPacketSequenceNumber sequence_number,
                             QuicTime::Delta delta_largest_observed) {
   if (unacked_packets_ <= 0) {
-    LOG(DFATAL) << "Acked more packets than were tracked.";
+    LOG(DFATAL) << "Acked more packets than were tracked."
+                << " unacked_packets:" << unacked_packets_;
     return true;
   }
   --unacked_packets_;
-  if (IsEmpty()) {
+  if (!HasUnackedPackets()) {
     // We have seen all the sequence numbers we were waiting for, trigger
     // callback notification.
     delegate_->OnAckNotification(retransmitted_packet_count_,
@@ -51,6 +52,16 @@ bool QuicAckNotifier::OnAck(QuicPacketSequenceNumber sequence_number,
   return false;
 }
 
+bool QuicAckNotifier::OnPacketAbandoned() {
+  if (unacked_packets_ <= 0) {
+    LOG(DFATAL) << "Abandoned more packets than were tracked."
+                << " unacked_packets:" << unacked_packets_;
+    return true;
+  }
+  --unacked_packets_;
+  return unacked_packets_ == 0;
+}
+
 void QuicAckNotifier::OnPacketRetransmitted(int packet_payload_size) {
   ++retransmitted_packet_count_;
   retransmitted_byte_count_ += packet_payload_size;