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

Source/platform/TaskSynchronizer.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 30 matching lines @@
     WTF_MAKE_NONCOPYABLE(TaskSynchronizer);
 public:
     TaskSynchronizer();
 
     // Called from a thread that waits for the task completion.
     void waitForTaskCompletion();
 
     // Called from a thread that executes the task.
     void taskCompleted();
 
+#if ENABLE(ASSERT)
+    bool hasCheckedForTermination() const { return m_hasCheckedForTermination; }
+    void setHasCheckedForTermination() { m_hasCheckedForTermination = true; }
+#endif
+
 private:
     void waitForTaskCompletionInternal();
 
     bool m_taskCompleted;
     Mutex m_synchronousMutex;
     ThreadCondition m_synchronousCondition;
+#if ENABLE(ASSERT)
+    bool m_hasCheckedForTermination;
+#endif
 };
 
 } // namespace blink
 
 #endif // TaskSynchronizer_h