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

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 8084b335ff8be86090c3c31e88a9be8306303b86..3364b142661f018a8ef783d0f05d7ca84c01fffd 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -350,11 +350,11 @@ public:
     {
         return adoptPtr(new AutofocusTask());
     }
-    virtual ~AutofocusTask() { }
+    ~AutofocusTask() override { }
 
 private:
     AutofocusTask() { }
-    virtual void performTask(ExecutionContext* context) override
+    void performTask(ExecutionContext* context) override
     {
         Document* document = toDocument(context);
         if (RefPtrWillBeRawPtr<Element> element = document->autofocusElement()) {