Standardize usage of virtual/override/final specifiers.

ui/views/examples/checkbox_example.h

Index: ui/views/examples/checkbox_example.h
diff --git a/ui/views/examples/checkbox_example.h b/ui/views/examples/checkbox_example.h
index d95f1f40cb0197f7dfd2820683505439f9dc8e9c..9e1013454936842c69ca74bafe741477fefb0cc0 100644
--- a/ui/views/examples/checkbox_example.h
+++ b/ui/views/examples/checkbox_example.h
@@ -19,14 +19,14 @@ class VIEWS_EXAMPLES_EXPORT CheckboxExample : public ExampleBase,
                                               public ButtonListener {
  public:
   CheckboxExample();
-  virtual ~CheckboxExample();
+  ~CheckboxExample() override;
 
   // ExampleBase:
-  virtual void CreateExampleView(View* container) override;
+  void CreateExampleView(View* container) override;
 
  private:
   // ButtonListener:
-  virtual void ButtonPressed(Button* sender, const ui::Event& event) override;
+  void ButtonPressed(Button* sender, const ui::Event& event) override;
 
   // The only control in this test.
   Checkbox* button_;