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

Source/platform/TaskSynchronizer.h

Index: Source/platform/TaskSynchronizer.h
diff --git a/Source/platform/TaskSynchronizer.h b/Source/platform/TaskSynchronizer.h
index dd279a6a66072e81133c3df66f70c3907c8f274d..9ffa0eb43fab75ba56dfb252cea85ff41fbc0c66 100644
--- a/Source/platform/TaskSynchronizer.h
+++ b/Source/platform/TaskSynchronizer.h
@@ -48,12 +48,20 @@
     // 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