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

components/user_manager/empty_user_info.cc

 // Copyright 2014 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 "components/user_manager/empty_user_info.h"
 
 #include "base/logging.h"
 #include "base/strings/utf_string_conversions.h"
 
 namespace user_manager {
 
-EmptyUserInfo::EmptyUserInfo() {
+EmptyUserInfo::EmptyUserInfo() : user_id_(std::string() /* gaia_id */, std::string() /* email */ ) {
 }
 
 EmptyUserInfo::~EmptyUserInfo() {
 }
 
 base::string16 EmptyUserInfo::GetDisplayName() const {
   NOTIMPLEMENTED();
   return base::string16();
 }
 
 base::string16 EmptyUserInfo::GetGivenName() const {
   NOTIMPLEMENTED();
   return base::string16();
 }
 
 std::string EmptyUserInfo::GetEmail() const {
   NOTIMPLEMENTED();
   return std::string();
 }
 
-std::string EmptyUserInfo::GetUserID() const {
+const UserID& EmptyUserInfo::GetUserID() const {
   NOTIMPLEMENTED();
-  return std::string();
+  return user_id_;
 }
 
 const gfx::ImageSkia& EmptyUserInfo::GetImage() const {
   NOTIMPLEMENTED();
   // To make the compiler happy.
   return null_image_;
 }
 
 }  // namespace user_manager