Revert of Remove the concept of a cleanup task

Source/modules/webdatabase/DatabaseTracker.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 100 matching lines @@
     static PassOwnPtr<NotifyDatabaseObserverOnCloseTask> create(Database* database)
     {
         return adoptPtr(new NotifyDatabaseObserverOnCloseTask(database));
     }
 
     virtual void performTask(ExecutionContext*) override
     {
         databaseClosed(m_database.get());
     }
 
+    virtual bool isCleanupTask() const override
+    {
+        return true;
+    }
+
 private:
     explicit NotifyDatabaseObserverOnCloseTask(Database* database)
         : m_database(database)
     {
     }
 
     CrossThreadPersistent<Database> m_database;
 };
 
 void DatabaseTracker::removeOpenDatabase(Database* database)
@@ skipping 125 matching lines @@
         DatabaseSet::iterator found = databaseSet->find(database);
         if (found == databaseSet->end())
             return;
     }
 
     // And we have to call closeImmediately() without our collection lock being held.
     database->closeImmediately();
 }
 
 } // namespace blink