Remove an unneeded hash_set from QuicAckNotifierManager.

net/quic/quic_ack_notifier.h

Index: net/quic/quic_ack_notifier.h
diff --git a/net/quic/quic_ack_notifier.h b/net/quic/quic_ack_notifier.h
index 10082ee67269f2281a458bf9224d2d7def5e4e95..b9b1cc67a5e06a73c13a68efbb01e3f2e4a19c9a 100644
--- a/net/quic/quic_ack_notifier.h
+++ b/net/quic/quic_ack_notifier.h
@@ -54,7 +54,12 @@ class NET_EXPORT_PRIVATE QuicAckNotifier {
   bool OnAck(QuicPacketSequenceNumber sequence_number,
              QuicTime::Delta delta_largest_observed);
 
-  bool IsEmpty() { return unacked_packets_ == 0; }
+  // Called when we've given up waiting for a sequence number, typically when
+  // the connection is torn down.
+  // Returns true if there are no more unacked packets being tracked.
+  bool OnPacketAbandoned();
+
+  bool HasUnackedPackets() const { return unacked_packets_ > 0; }
 
   // If a packet is retransmitted by the connection, it will be sent with a
   // different sequence number.