Revamp the MigrateBrowserPrefs and MigrateUserPrefs code.

chrome/browser/chromeos/login/default_pinned_apps_field_trial.h

 // Copyright 2013 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 CHROME_BROWSER_CHROMEOS_LOGIN_DEFAULT_PINNED_APPS_FIELD_TRIAL_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_DEFAULT_PINNED_APPS_FIELD_TRIAL_H_
 
 #include <string>
 
 namespace base {
 class ListValue;
 }
 
 class PrefRegistrySimple;
 class PrefService;
 
 namespace chromeos {
 namespace default_pinned_apps_field_trial {
 
 // Registers a local state that keeps track of users in experiment.
 void RegisterPrefs(PrefRegistrySimple* registry);
 
-// Migrate by removing the old local state.
-void MigratePrefs(PrefService* local_state);
 
 }  // namespace default_pinned_apps_field_trial
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_DEFAULT_PINNED_APPS_FIELD_TRIAL_H_