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

ash/system/user/tray_user.cc

 // Copyright (c) 2012 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 "ash/system/user/tray_user.h"
 
 #include "ash/ash_switches.h"
 #include "ash/root_window_controller.h"
 #include "ash/session/session_state_delegate.h"
 #include "ash/shelf/shelf_layout_manager.h"
 #include "ash/shell_delegate.h"
 #include "ash/system/tray/system_tray.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "ash/system/tray/system_tray_notifier.h"
 #include "ash/system/tray/tray_constants.h"
 #include "ash/system/tray/tray_item_view.h"
 #include "ash/system/tray/tray_utils.h"
 #include "ash/system/user/accounts_detailed_view.h"
 #include "ash/system/user/rounded_image_view.h"
 #include "ash/system/user/user_view.h"
 #include "base/logging.h"
 #include "base/strings/string16.h"
+#include "components/user_manager/user_id.h"
 #include "components/user_manager/user_info.h"
 #include "grit/ash_strings.h"
 #include "ui/aura/window.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/gfx/image/image.h"
 #include "ui/views/border.h"
 #include "ui/views/controls/label.h"
 #include "ui/views/layout/box_layout.h"
 #include "ui/views/view.h"
 #include "ui/views/widget/widget.h"
@@ skipping 73 matching lines @@
 
   // Do not show more UserView's then there are logged in users.
   if (multiprofile_index_ >= logged_in_users)
     return nullptr;
 
   user_ = new tray::UserView(this, status, multiprofile_index_, false);
   return user_;
 }
 
 views::View* TrayUser::CreateDetailedView(user::LoginStatus status) {
-  std::string user_id = Shell::GetInstance()
+  const user_manager::UserID user_id = Shell::GetInstance()
                             ->session_state_delegate()
                             ->GetUserInfo(0)
                             ->GetUserID();
   tray::UserAccountsDelegate* delegate =
       Shell::GetInstance()->system_tray_delegate()->GetUserAccountsDelegate(
           user_id);
   if (!delegate)
     return nullptr;
   return new tray::AccountsDetailedView(this, status, delegate);
 }
@@ skipping 164 matching lines @@
 void TrayUser::UpdateLayoutOfItem() {
   RootWindowController* controller = GetRootWindowController(
       system_tray()->GetWidget()->GetNativeWindow()->GetRootWindow());
   if (controller && controller->shelf()) {
     UpdateAfterShelfAlignmentChange(
         controller->GetShelfLayoutManager()->GetAlignment());
   }
 }
 
 }  // namespace ash