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

Source/core/dom/StringCallback.cpp

Index: Source/core/dom/StringCallback.cpp
diff --git a/Source/core/dom/StringCallback.cpp b/Source/core/dom/StringCallback.cpp
index 669a442947119c40f65df55bb9fef5a8aabd6e15..8933dfb5f82de85d84e7256ce181ea433453af71 100644
--- a/Source/core/dom/StringCallback.cpp
+++ b/Source/core/dom/StringCallback.cpp
@@ -47,12 +47,12 @@ public:
         return adoptPtr(new DispatchCallbackTask(callback, data, taskName));
     }
 
-    virtual void performTask(ExecutionContext*) override
+    void performTask(ExecutionContext*) override
     {
         m_callback->handleEvent(m_data);
     }
 
-    virtual String taskNameForInstrumentation() const override
+    String taskNameForInstrumentation() const override
     {
         return m_taskName;
     }