Revert of DatabaseBackend should not post a task to an already terminated database thread

Source/modules/webdatabase/DatabaseTask.h

 /*
  * Copyright (C) 2007, 2008, 2013 Apple 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 38 matching lines @@
 class DatabaseTask : public WebThread::Task {
     WTF_MAKE_NONCOPYABLE(DatabaseTask); WTF_MAKE_FAST_ALLOCATED;
 public:
     virtual ~DatabaseTask();
 
     virtual void run() OVERRIDE FINAL;
 
     Database* database() const { return m_database.get(); }
 #if ENABLE(ASSERT)
     bool hasSynchronizer() const { return m_synchronizer; }
+    bool hasCheckedForTermination() const { return m_synchronizer->hasCheckedForTermination(); }
 #endif
 
 protected:
     DatabaseTask(Database*, TaskSynchronizer*);
 
 private:
     virtual void doPerformTask() = 0;
     virtual void taskCancelled() { }
 
     RefPtrWillBeCrossThreadPersistent<Database> m_database;
@@ skipping 80 matching lines @@
 #if !LOG_DISABLED
     virtual const char* debugTaskName() const OVERRIDE;
 #endif
 
     Vector<String>& m_tableNames;
 };
 
 } // namespace blink
 
 #endif // DatabaseTask_h