Standardize usage of virtual/override/final specifiers.

ash/wm/screen_dimmer.h

Index: ash/wm/screen_dimmer.h
diff --git a/ash/wm/screen_dimmer.h b/ash/wm/screen_dimmer.h
index 489b93345beb837493ec761138d175d2d48e6dc5..a0b3fa57b8d2d67c15a2cf0c8ab933f427901c3d 100644
--- a/ash/wm/screen_dimmer.h
+++ b/ash/wm/screen_dimmer.h
@@ -39,15 +39,15 @@ class ASH_EXPORT ScreenDimmer : public aura::WindowObserver {
   };
 
   explicit ScreenDimmer(aura::Window* root_window);
-  virtual ~ScreenDimmer();
+  ~ScreenDimmer() override;
 
   // Dim or undim the root window.
   void SetDimming(bool should_dim);
 
   // aura::WindowObserver overrides:
-  virtual void OnWindowBoundsChanged(aura::Window* root_window,
-                                     const gfx::Rect& old_bounds,
-                                     const gfx::Rect& new_bounds) override;
+  void OnWindowBoundsChanged(aura::Window* root_window,
+                             const gfx::Rect& old_bounds,
+                             const gfx::Rect& new_bounds) override;
 
  private:
   friend class TestApi;