Fix virtual/override/final usage in Source/core/dom/.

Source/core/dom/MainThreadTaskRunnerTest.cpp

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * 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:
  * 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 29 matching lines @@
 namespace blink {
 
 class MarkingBooleanTask final : public ExecutionContextTask {
 public:
     static PassOwnPtr<MarkingBooleanTask> create(bool* toBeMarked)
     {
         return adoptPtr(new MarkingBooleanTask(toBeMarked));
     }
 
 
-    virtual ~MarkingBooleanTask() { }
+    ~MarkingBooleanTask() override { }
 
 private:
     MarkingBooleanTask(bool* toBeMarked) : m_toBeMarked(toBeMarked) { }
 
-    virtual void performTask(ExecutionContext* context) override
+    void performTask(ExecutionContext* context) override
     {
         *m_toBeMarked = true;
     }
 
     bool* m_toBeMarked;
 };
 
 TEST(MainThreadTaskRunnerTest, PostTask)
 {
     RefPtrWillBeRawPtr<NullExecutionContext> context = adoptRefWillBeNoop(new NullExecutionContext());
@@ skipping 31 matching lines @@
     bool isMarked = false;
 
     context->setTasksNeedSuspension(true);
     runner->postTask(FROM_HERE, MarkingBooleanTask::create(&isMarked));
     runner.clear();
     blink::testing::runPendingTasks();
     EXPECT_FALSE(isMarked);
 }
 
 } // namespace blink