Introduce PrefRegistrySyncable, simplifying PrefServiceSyncable.

chrome/chrome_browser.gypi

 # Copyright 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.
 
 {
   'targets': [
     {
       'target_name': 'browser',
       'type': 'static_library',
       'variables': { 'enable_wexit_time_destructors': 1, },
@@ skipping 1470 matching lines @@
         'browser/prefs/incognito_mode_prefs.cc',
         'browser/prefs/incognito_mode_prefs.h',
         'browser/prefs/pref_model_associator.cc',
         'browser/prefs/pref_model_associator.h',
         'browser/prefs/pref_notifier_impl.cc',
         'browser/prefs/pref_notifier_impl.h',
         'browser/prefs/pref_registry.cc',
         'browser/prefs/pref_registry.h',
         'browser/prefs/pref_registry_simple.cc',
         'browser/prefs/pref_registry_simple.h',
+        'browser/prefs/pref_registry_syncable.cc',
+        'browser/prefs/pref_registry_syncable.h',
         'browser/prefs/pref_service.cc',
         'browser/prefs/pref_service.h',
         'browser/prefs/pref_service_builder.cc',
         'browser/prefs/pref_service_builder.h',
         'browser/prefs/pref_service_syncable.cc',
         'browser/prefs/pref_service_syncable.h',
         'browser/prefs/pref_service_syncable_builder.cc',
         'browser/prefs/pref_service_syncable_builder.h',
         'browser/prefs/pref_service_syncable_observer.h',
         'browser/prefs/pref_value_store.cc',
@@ skipping 1627 matching lines @@
           'variables': {
             'jni_gen_dir': 'chrome',
           },
           'includes': [ '../build/jni_generator.gypi' ],
         },
       ],
      },
     ],
   ],
 }