Revert of Remove the concept of a cleanup task

Source/core/dom/ExecutionContextTask.h

Index: Source/core/dom/ExecutionContextTask.h
diff --git a/Source/core/dom/ExecutionContextTask.h b/Source/core/dom/ExecutionContextTask.h
index e39ab562eb5967e42cdf8e321a30f6b10a653fee..9f1f5d19b22745f41e1380aff027d8d6efc4f1ac 100644
--- a/Source/core/dom/ExecutionContextTask.h
+++ b/Source/core/dom/ExecutionContextTask.h
@@ -45,6 +45,8 @@
     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(); }
 };