relnote: Add num_connections_ in quic_time_wait_list_manager to record

net/tools/quic/quic_time_wait_list_manager.h

Index: net/tools/quic/quic_time_wait_list_manager.h
diff --git a/net/tools/quic/quic_time_wait_list_manager.h b/net/tools/quic/quic_time_wait_list_manager.h
index 9a9eccd5e78fb9fd530d84805574dabcb09b6657..c2368dc1553466dfec9528365e88cbc228219c56 100644
--- a/net/tools/quic/quic_time_wait_list_manager.h
+++ b/net/tools/quic/quic_time_wait_list_manager.h
@@ -101,7 +101,7 @@ class QuicTimeWaitListManager : public QuicBlockedWriterInterface {
   QuicVersion GetQuicVersionFromConnectionId(QuicConnectionId connection_id);
 
   // The number of connections on the time-wait list.
-  size_t num_connections() const { return connection_id_map_.size(); }
+  size_t num_connections() const { return num_connections_; }
 
  protected:
   virtual QuicEncryptedPacket* BuildPublicReset(
@@ -190,6 +190,12 @@ class QuicTimeWaitListManager : public QuicBlockedWriterInterface {
   // Interface that manages blocked writers.
   QuicServerSessionVisitor* visitor_;
 
+  // Number of connection IDs in |connection_id_map_|. According to b/23531792,
+  // cost of size() of linked_hash_map is linear instead of constant, and a
+  // separate counter is maintained so that cost of num_connections() is
+  // constant. TODO(b/23531792): Remove this counter.
+  size_t num_connections_;
+
   DISALLOW_COPY_AND_ASSIGN(QuicTimeWaitListManager);
 };