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

Source/core/dom/MainThreadTaskRunnerTest.cpp

Index: Source/core/dom/MainThreadTaskRunnerTest.cpp
diff --git a/Source/core/dom/MainThreadTaskRunnerTest.cpp b/Source/core/dom/MainThreadTaskRunnerTest.cpp
index cb1d63da9f02301a02a389592ba630e031d1fc43..0a643596747e82a310325643c408eb9b725c8c37 100644
--- a/Source/core/dom/MainThreadTaskRunnerTest.cpp
+++ b/Source/core/dom/MainThreadTaskRunnerTest.cpp
@@ -47,12 +47,12 @@ public:
     }
 
 
-    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;
     }