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

chrome/browser/ui/ash/multi_user/multi_user_window_manager_stub.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/ui/ash/multi_user/multi_user_window_manager_stub.h"
 
 #include "base/logging.h"
 #include "base/strings/string_util.h"
+#include "components/user_manager/user_id.h"
 
 namespace chrome {
 
 void MultiUserWindowManagerStub::SetWindowOwner(aura::Window* window,
-                                               const std::string& user_id) {
+                                               const user_manager::UserID& user_id) {
   NOTIMPLEMENTED();
 }
 
-const std::string& MultiUserWindowManagerStub::GetWindowOwner(
+const user_manager::UserID& MultiUserWindowManagerStub::GetWindowOwner(
     aura::Window* window) const {
-  return base::EmptyString();
+  return user_manager::EmptyUserID();
 }
 
 void MultiUserWindowManagerStub::ShowWindowForUser(aura::Window* window,
-                                                  const std::string& user_id) {
+                                                  const user_manager::UserID& user_id) {
   NOTIMPLEMENTED();
 }
 
 bool MultiUserWindowManagerStub::AreWindowsSharedAmongUsers() const {
   return false;
 }
 
 void MultiUserWindowManagerStub::GetOwnersOfVisibleWindows(
-    std::set<std::string>* user_ids) const {
+    std::set<user_manager::UserID>* user_ids) const {
 }
 
 bool MultiUserWindowManagerStub::IsWindowOnDesktopOfUser(
     aura::Window* window,
-    const std::string& user_id) const {
+    const user_manager::UserID& user_id) const {
   return true;
 }
 
-const std::string& MultiUserWindowManagerStub::GetUserPresentingWindow(
+const user_manager::UserID& MultiUserWindowManagerStub::GetUserPresentingWindow(
     aura::Window* window) const {
-  return base::EmptyString();
+  return user_manager::EmptyUserID();
 }
 
 void MultiUserWindowManagerStub::AddUser(content::BrowserContext* context) {
   NOTIMPLEMENTED();
 }
 
 void MultiUserWindowManagerStub::AddObserver(Observer* observer) {
   NOTIMPLEMENTED();
 }
 
 void MultiUserWindowManagerStub::RemoveObserver(Observer* observer) {
   NOTIMPLEMENTED();
 }
 
 }  // namespace chrome