GTTF: Add missing virtual destructors.

base/prefs/pref_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 BASE_PREFS_PREF_OBSERVER_H_
 #define BASE_PREFS_PREF_OBSERVER_H_
 
 #include <string>
 
 class PrefServiceBase;
 
 // TODO(joi): Switch to base::Callback and remove this.
 class PrefObserver {
  public:
   virtual void OnPreferenceChanged(PrefServiceBase* service,
                                    const std::string& pref_name) = 0;
+
+ protected:
+  virtual ~PrefObserver() { }
 };
 
 #endif  // BASE_PREFS_PREF_OBSERVER_H_