C++11 override style change for athena

athena/system/shutdown_dialog.h

Index: athena/system/shutdown_dialog.h
diff --git a/athena/system/shutdown_dialog.h b/athena/system/shutdown_dialog.h
index fb8d1686619eb7a9b49b66b8b0887085ccb915c1..8409710ad4c31345a622a023a08cea8c3206fdbd 100644
--- a/athena/system/shutdown_dialog.h
+++ b/athena/system/shutdown_dialog.h
@@ -24,7 +24,7 @@ namespace athena {
 class ShutdownDialog : public PowerButtonObserver {
  public:
   explicit ShutdownDialog();
-  virtual ~ShutdownDialog();
+  ~ShutdownDialog() override;
 
  private:
   enum State {
@@ -48,8 +48,7 @@ class ShutdownDialog : public PowerButtonObserver {
   void Shutdown();
 
   // PowerButtonObserver:
-  virtual void OnPowerButtonStateChanged(
-      PowerButtonObserver::State state) override;
+  void OnPowerButtonStateChanged(PowerButtonObserver::State state) override;
 
   // |shutdown_warning_message_|'s parent container.
   aura::Window* warning_message_container_;