We should use UserID object to identify users instead of username.

chrome/browser/chromeos/app_mode/kiosk_app_data.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/app_mode/kiosk_app_data.h"
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/files/file_util.h"
@@ skipping 351 matching lines @@
   base::WeakPtr<KioskAppData> client_;
 
   DISALLOW_COPY_AND_ASSIGN(WebstoreDataParser);
 };
 
 ////////////////////////////////////////////////////////////////////////////////
 // KioskAppData
 
 KioskAppData::KioskAppData(KioskAppDataDelegate* delegate,
                            const std::string& app_id,
-                           const std::string& user_id,
+                           const user_manager::UserID& user_id,
                            const GURL& update_url)
     : delegate_(delegate),
       status_(STATUS_INIT),
       app_id_(app_id),
       user_id_(user_id),
       update_url_(update_url) {
 }
 
 KioskAppData::~KioskAppData() {}
 
@@ skipping 271 matching lines @@
 
   SkBitmap icon = crx_loader->icon();
   if (icon.empty())
     icon = *extensions::util::GetDefaultAppIcon().bitmap();
   SetCache(crx_loader->name(), icon);
 
   SetStatus(STATUS_LOADED);
 }
 
 }  // namespace chromeos