Fix NetworkSmsHandler to observe Manager and improve API

chromeos/dbus/shill_property_changed_observer.h

Index: chromeos/dbus/shill_property_changed_observer.h
diff --git a/chromeos/dbus/shill_property_changed_observer.h b/chromeos/dbus/shill_property_changed_observer.h
index d27482eb8ecf5c64a70f97096e90a731ed24f314..7ee937328cbddc7ea0930034c6d4c07d29a9253b 100644
--- a/chromeos/dbus/shill_property_changed_observer.h
+++ b/chromeos/dbus/shill_property_changed_observer.h
@@ -17,9 +17,11 @@ namespace chromeos {
 // 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