Introduce PrefRegistrySyncable, simplifying PrefServiceSyncable.

chrome/browser/bookmarks/DEPS

 include_rules = [
   # Bookmarks is being made into a Browser Component, so we have these
   # two basic rules followed by temporary exceptions.  Please don't
   # add to the list of exceptions!
   "-chrome/browser",
   "+chrome/browser/api",
   "+chrome/browser/bookmarks",
   "+chrome/browser/common",
 
   # TODO(kaiwang): Bring this list to zero.
   # Do not add to the list of temporarily-allowed dependencies below,
   # and please do not introduce more #includes of these files.
   "!chrome/browser/favicon/favicon_service.h",
   "!chrome/browser/favicon/favicon_service_factory.h",
   "!chrome/browser/history/history_notifications.h",
   "!chrome/browser/history/history_service.h",
   "!chrome/browser/history/history_service_factory.h",
   "!chrome/browser/history/history_types.h",
   "!chrome/browser/history/query_parser.h",
   "!chrome/browser/history/snippet.h",
   "!chrome/browser/history/url_database.h",
-  "!chrome/browser/prefs/pref_service_syncable.h",
+  "!chrome/browser/prefs/pref_registry_syncable.h",
+  "!chrome/browser/prefs/pref_service.h",
   "!chrome/browser/profiles/profile.h",
   "!chrome/browser/profiles/profile_dependency_manager.h",
   "!chrome/browser/profiles/profile_keyed_service.h",
   "!chrome/browser/profiles/profile_keyed_service_factory.h",
   # Do not add to the list of temporarily-allowed dependencies above,
   # and please do not introduce more #includes of these files.
 ]
 
 specific_include_rules = {
   # TODO(kaiwang): Bring this list to zero.
   # Do not add to the list of temporarily-allowed dependencies below,
   # and please do not introduce more #includes of these files.
   '.*_[a-z]*test\.cc': [
     "!chrome/browser/history/in_memory_database.h",
     "!chrome/browser/importer/firefox2_importer.h",
   ]
 }