Revert of Remove the concept of a cleanup task

Source/core/dom/ExecutionContextTask.h

 /*
  * Copyright (C) 2013 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.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 27 matching lines @@
 
 class ExecutionContext;
 
 class CORE_EXPORT ExecutionContextTask {
     WTF_MAKE_NONCOPYABLE(ExecutionContextTask);
     WTF_MAKE_FAST_ALLOCATED(ExecutionContextTask);
 public:
     ExecutionContextTask() { }
     virtual ~ExecutionContextTask() { }
     virtual void performTask(ExecutionContext*) = 0;
+    // Certain tasks get marked specially so that they aren't discarded, and are executed, when the context is shutting down its message queue.
+    virtual bool isCleanupTask() const { return false; }
     virtual String taskNameForInstrumentation() const { return String(); }
 };
 
 namespace internal {
 
 template<typename T>
 class CallClosureTaskBase : public ExecutionContextTask {
 protected:
     CallClosureTaskBase(PassOwnPtr<Function<T>> closure, bool isSameThread)
         : m_closure(closure)
@@ skipping 55 matching lines @@
 template<typename FunctionType, typename... P>
 PassOwnPtr<ExecutionContextTask> createSameThreadTask(
     FunctionType function, const P&... parameters)
 {
     return internal::CallClosureTask::create(bind(function, parameters...), true);
 }
 
 } // namespace
 
 #endif