[iOS] Remove support for asynchronous pref creation.

ios/chrome/browser/prefs/ios_chrome_pref_service_factory.h

 // Copyright 2015 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 IOS_CHROME_BROWSER_PREFS_IOS_CHROME_PREF_SERVICE_FACTORY_H_
 #define IOS_CHROME_BROWSER_PREFS_IOS_CHROME_PREF_SERVICE_FACTORY_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 
@@ skipping 16 matching lines @@
 }
 
 namespace user_prefs {
 class PrefRegistrySyncable;
 }
 
 // Factory methods that create and initialize a new instance of a PrefService
 // for Chrome on iOS with the applicable PrefStores. The |pref_filename| points
 // to the user preference file. This is the usual way to create a new
 // PrefService. |pref_registry| keeps the list of registered prefs and their
-// default valuers. If |async| is true, asynchronous version is used.
+// default values.
 scoped_ptr<PrefService> CreateLocalState(
     const base::FilePath& pref_filename,
     base::SequencedTaskRunner* pref_io_task_runner,
-    const scoped_refptr<PrefRegistry>& pref_registry,
-    bool async);
+    const scoped_refptr<PrefRegistry>& pref_registry);
 
 scoped_ptr<syncable_prefs::PrefServiceSyncable> CreateBrowserStatePrefs(
     const base::FilePath& browser_state_path,
     base::SequencedTaskRunner* pref_io_task_runner,
     TrackedPreferenceValidationDelegate* validation_delegate,
-    const scoped_refptr<user_prefs::PrefRegistrySyncable>& pref_registry,
-    bool async);
+    const scoped_refptr<user_prefs::PrefRegistrySyncable>& pref_registry);
 
 // Creates an incognito copy of |pref_service| that shares most prefs but uses
 // a fresh non-persistent overlay for the user pref store.
 scoped_ptr<syncable_prefs::PrefServiceSyncable>
 CreateIncognitoBrowserStatePrefs(
     syncable_prefs::PrefServiceSyncable* main_pref_store);
 
 #endif  // IOS_CHROME_BROWSER_PREFS_IOS_CHROME_PREF_SERVICE_FACTORY_H_