Add undeclared virtual destructors part 2...

app/active_window_watcher_x.h

 // Copyright (c) 2009 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 APP_ACTIVE_WINDOW_WATCHER_X_H_
 #define APP_ACTIVE_WINDOW_WATCHER_X_H_
 
 #include <gdk/gdk.h>
 
 #include "base/basictypes.h"
 #include "base/observer_list.h"
 #include "base/singleton.h"
 
 // This is a helper class that is used to keep track of which window the X
 // window manager thinks is active. Add an Observer to listener for changes to
 // the active window.
 class ActiveWindowWatcherX {
  public:
   class Observer {
    public:
     // |active_window| will be NULL if the active window isn't one of Chrome's.
     virtual void ActiveWindowChanged(GdkWindow* active_window) = 0;
+
+   protected:
+    virtual ~Observer() {}
   };
 
   static void AddObserver(Observer* observer);
   static void RemoveObserver(Observer* observer);
 
  private:
   friend struct DefaultSingletonTraits<ActiveWindowWatcherX>;
 
   ActiveWindowWatcherX();
 
   void Init();
 
   // Sends a notification out through the NotificationService that the active
   // window has changed.
   void NotifyActiveWindowChanged();
 
   // Callback for PropertyChange XEvents.
   static GdkFilterReturn OnWindowXEvent(GdkXEvent* xevent,
                                         GdkEvent* event,
                                         gpointer window_watcher);
 
   ObserverList<Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ActiveWindowWatcherX);
 };
 
 #endif  // APP_ACTIVE_WINDOW_WATCHER_X_H_