Update {virtual,override,final} to follow C++11 style in ash.

ash/system/chromeos/session/logout_button_tray.cc

Index: ash/system/chromeos/session/logout_button_tray.cc
diff --git a/ash/system/chromeos/session/logout_button_tray.cc b/ash/system/chromeos/session/logout_button_tray.cc
index 139623108c8e1d04b676a069a2a907d548199bfa..084af0a3bf98e4f7303620b0383543b3586565b8 100644
--- a/ash/system/chromeos/session/logout_button_tray.cc
+++ b/ash/system/chromeos/session/logout_button_tray.cc
@@ -56,7 +56,7 @@ const int kLogoutButtonPushedImages[] = {
 class LogoutButton : public views::LabelButton {
  public:
   LogoutButton(views::ButtonListener* listener);
-  virtual ~LogoutButton();
+  ~LogoutButton() override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LogoutButton);