[S] Use a weak_factory for V4Database callbacks + test

components/safe_browsing_db/v4_database.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_SAFE_BROWSING_DB_V4_DATABASE_H_
 #define COMPONENTS_SAFE_BROWSING_DB_V4_DATABASE_H_
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
 #include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
 #include "components/safe_browsing_db/v4_protocol_manager_util.h"
 #include "components/safe_browsing_db/v4_store.h"
 
 namespace safe_browsing {
 
 class V4Database;
 
 // Scheduled when the database has been read from disk and is ready to process
@@ skipping 177 matching lines @@
 
   // The factory that controls the creation of V4Store objects.
   static V4StoreFactory* factory_;
 
   // The number of stores for which the update request is pending. When this
   // goes down to 0, that indicates that the database has updated all the stores
   // that needed updating and is ready for the next update. It should only be
   // accessed on the IO thread.
   int pending_store_updates_;
 
+  // Only meant to be dereferenced and invalidated on the IO thread and hence
+  // named. For details, see the comment at the top of weak_ptr.h
+  base::WeakPtrFactory<V4Database> weak_factory_on_io_;
+
   DISALLOW_COPY_AND_ASSIGN(V4Database);
 };
 
 }  // namespace safe_browsing
 
 #endif  // COMPONENTS_SAFE_BROWSING_DB_V4_DATABASE_H_