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

Source/platform/TaskSynchronizer.cpp

 /*
  * 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 17 matching lines @@
 
 #include "config.h"
 #include "platform/TaskSynchronizer.h"
 
 #include "heap/ThreadState.h"
 
 namespace blink {
 
 TaskSynchronizer::TaskSynchronizer()
     : m_taskCompleted(false)
+#if ENABLE(ASSERT)
+    , m_hasCheckedForTermination(false)
+#endif
 {
 }
 
 void TaskSynchronizer::waitForTaskCompletion()
 {
     if (ThreadState::current()) {
         // Prevent the deadlock between park request by other threads and blocking
         // by m_synchronousCondition.
         ThreadState::SafePointScope scope(ThreadState::HeapPointersOnStack);
         waitForTaskCompletionInternal();
@@ skipping 13 matching lines @@
 
 void TaskSynchronizer::taskCompleted()
 {
     m_synchronousMutex.lock();
     m_taskCompleted = true;
     m_synchronousCondition.signal();
     m_synchronousMutex.unlock();
 }
 
 } // namespace blink