Introduce PrefRegistrySyncable, simplifying PrefServiceSyncable.

chrome/browser/prefs/pref_service.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.
 
 // This provides a way to access the application's current preferences.
 
 // Chromium settings and storage represent user-selected preferences and
 // information and MUST not be extracted, overwritten or modified except
 // through Chromium defined APIs.
 
@@ skipping 163 matching lines @@
   // passed to us at construction time.
   void AddInitialPreferences();
 
   // Updates local caches for a preference registered at |path|. The
   // |default_value| must not be NULL as it determines the preference
   // value's type.  AddRegisteredPreference must not be called twice
   // for the same path.
   void AddRegisteredPreference(const char* path,
                                base::Value* default_value);
 
-  void RemoveRegisteredPreference(const char* path);
+  // Updates local caches for a preference unregistered at |path|.
+  virtual void RemoveRegisteredPreference(const char* path);
 
   // The PrefNotifier handles registering and notifying preference observers.
   // It is created and owned by this PrefService. Subclasses may access it for
   // unit testing.
   scoped_ptr<PrefNotifierImpl> pref_notifier_;
 
   // The PrefValueStore provides prioritized preference values. It is owned by
   // this PrefService. Subclasses may access it for unit testing.
   scoped_ptr<PrefValueStore> pref_value_store_;
 
@@ skipping 57 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(PrefService);
 };
 
 // TODO(joi): Remove these forwards. They were placed here temporarily
 // to limit the size of the initial change that split
 // PrefServiceSimple and PrefServiceSyncable out of PrefService.
 #include "chrome/browser/prefs/pref_service_syncable.h"
 
 #endif  // CHROME_BROWSER_PREFS_PREF_SERVICE_H_