Add the "virtual" keyword on method overrides that are missing it.

chrome/browser/gtk/overflow_button.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_GTK_OVERFLOW_BUTTON_H_
 #define CHROME_BROWSER_GTK_OVERFLOW_BUTTON_H_
 #pragma once
 
 #include "chrome/browser/gtk/owned_widget_gtk.h"
 #include "chrome/common/notification_observer.h"
 #include "chrome/common/notification_registrar.h"
 
 typedef struct _GtkWidget GtkWidget;
 class Profile;
 
 // An overflow chevron button. The button itself is a plain gtk_chrome_button,
 // and this class handles theming it.
 class OverflowButton : public NotificationObserver {
  public:
   explicit OverflowButton(Profile* profile);
   virtual ~OverflowButton();
 
   GtkWidget* widget() { return widget_.get(); }
 
  private:
   // NotificationObserver implementation.
-  void Observe(NotificationType type,
-               const NotificationSource& source,
-               const NotificationDetails& details);
+  virtual void Observe(NotificationType type,
+                       const NotificationSource& source,
+                       const NotificationDetails& details);
 
   OwnedWidgetGtk widget_;
 
   Profile* profile_;
 
   NotificationRegistrar registrar_;
 };
 
 #endif  // CHROME_BROWSER_GTK_OVERFLOW_BUTTON_H_