Fix NetworkSmsHandler to observe Manager and improve API

chromeos/dbus/shill_property_changed_observer.h

 // Copyright (c) 2012 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 CHROMEOS_DBUS_SHILL_PROPERTY_CHANGED_OBSERVER_H_
 #define CHROMEOS_DBUS_SHILL_PROPERTY_CHANGED_OBSERVER_H_
 
 #include <string>
 
 namespace base {
 class Value;
 }
 
 namespace chromeos {
 
 // This is a base class for observers which handle the PropertyChanged signal
 // sent from Shill.
 class ShillPropertyChangedObserver {
  public:
-  virtual ~ShillPropertyChangedObserver() {}
   virtual void OnPropertyChanged(const std::string& name,
                                  const base::Value& value) = 0;
+
+ protected:
+  virtual ~ShillPropertyChangedObserver() {}
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_SHILL_PROPERTY_CHANGED_OBSERVER_H_