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

ash/system/tray/system_tray_delegate.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/tray/system_tray_delegate.h"
 
 namespace ash {
 
 NetworkIconInfo::NetworkIconInfo()
     : connecting(false),
@@ skipping 244 matching lines @@
 }
 
 void SystemTrayDelegate::ActiveUserWasChanged() {
 }
 
 bool SystemTrayDelegate::IsSearchKeyMappedToCapsLock() {
   return false;
 }
 
 tray::UserAccountsDelegate* SystemTrayDelegate::GetUserAccountsDelegate(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   return nullptr;
 }
 
 void SystemTrayDelegate::AddCustodianInfoTrayObserver(
     CustodianInfoTrayObserver* observer) {
 }
 
 void SystemTrayDelegate::RemoveCustodianInfoTrayObserver(
     CustodianInfoTrayObserver* observer) {
 }
 
 void SystemTrayDelegate::AddShutdownPolicyObserver(
     ShutdownPolicyObserver* observer) {
 }
 
 void SystemTrayDelegate::RemoveShutdownPolicyObserver(
     ShutdownPolicyObserver* observer) {
 }
 
 void SystemTrayDelegate::ShouldRebootOnShutdown(
     const RebootOnShutdownCallback& callback) {
 }
 
 VPNDelegate* SystemTrayDelegate::GetVPNDelegate() const {
   return nullptr;
 }
 
 }  // namespace ash