Protect AppListItemList Add/Remove and fix sync bugs

ui/app_list/app_list_switches.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 UI_APP_LIST_APP_LIST_SWITCHES_H_
 #define UI_APP_LIST_APP_LIST_SWITCHES_H_
 
 #include "ui/app_list/app_list_export.h"
 
 namespace app_list {
 namespace switches {
 
 APP_LIST_EXPORT extern const char kEnableFolderUI[];
 APP_LIST_EXPORT extern const char kDisableVoiceSearch[];
 
-bool IsFolderUIEnabled();
+bool APP_LIST_EXPORT IsFolderUIEnabled();
 
 bool APP_LIST_EXPORT IsVoiceSearchEnabled();
 
 }  // namespace switches
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_SWITCHES_H_