Standardize usage of virtual/override/final specifiers.

ash/shelf/overflow_button.h

Index: ash/shelf/overflow_button.h
diff --git a/ash/shelf/overflow_button.h b/ash/shelf/overflow_button.h
index 2dd79f287aef1c4435e53d2380027c6251781ede..2b75ceab77c62f365f6fdf181954fe7c336d34e2 100644
--- a/ash/shelf/overflow_button.h
+++ b/ash/shelf/overflow_button.h
@@ -17,7 +17,7 @@ namespace ash {
 class OverflowButton : public views::CustomButton {
  public:
   explicit OverflowButton(views::ButtonListener* listener);
-  virtual ~OverflowButton();
+  ~OverflowButton() override;
 
   void OnShelfAlignmentChanged();
 
@@ -25,7 +25,7 @@ class OverflowButton : public views::CustomButton {
   void PaintBackground(gfx::Canvas* canvas, int alpha);
 
   // views::View overrides:
-  virtual void OnPaint(gfx::Canvas* canvas) override;
+  void OnPaint(gfx::Canvas* canvas) override;
 
   // Left and right images are rotations of bottom_image and are
   // owned by the overflow button.