Revamp the MigrateBrowserPrefs and MigrateUserPrefs code.

chrome/browser/chromeos/login/default_pinned_apps_field_trial.cc

-// 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.
-
-#include "chrome/browser/chromeos/login/default_pinned_apps_field_trial.h"
-
-#include "base/basictypes.h"
-#include "base/prefs/pref_registry_simple.h"
-#include "base/prefs/pref_service.h"
-
-namespace chromeos {
-namespace default_pinned_apps_field_trial {
-
-namespace {
-
-// Name of a local state pref to store the list of users that participate
-// the experiment.
-const char kExperimentUsers[] = "DefaultPinnedAppsExperimentUsers";
-
-}  // namespace
-
-void RegisterPrefs(PrefRegistrySimple* registry) {
-  registry->RegisterListPref(kExperimentUsers);
-}
-
-void MigratePrefs(PrefService* local_state) {
-  // TODO(xiyuan): Remove everything in M34.
-  local_state->ClearPref(kExperimentUsers);
-}
-
-}  // namespace default_pinned_apps_field_trial
-}  // namespace chromeos