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

chrome/browser/ui/ash/multi_user/multi_user_window_manager_stub.h

 // 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.
 
 #ifndef CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_WINDOW_MANAGER_STUB_H_
 #define CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_WINDOW_MANAGER_STUB_H_
 
 #include <map>
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "chrome/browser/ui/ash/multi_user/multi_user_window_manager.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chrome {
 
 // This is the implementation of MultiUserWindowManager for single user mode.
 class MultiUserWindowManagerStub : public MultiUserWindowManager {
  public:
   MultiUserWindowManagerStub() {}
   ~MultiUserWindowManagerStub() override {}
 
   // MultiUserWindowManager overrides:
   void SetWindowOwner(aura::Window* window,
-                      const std::string& user_id) override;
-  const std::string& GetWindowOwner(aura::Window* window) const override;
+                      const user_manager::UserID& user_id) override;
+  const user_manager::UserID& GetWindowOwner(aura::Window* window) const override;
   void ShowWindowForUser(aura::Window* window,
-                         const std::string& user_id) override;
+                         const user_manager::UserID& user_id) override;
   bool AreWindowsSharedAmongUsers() const override;
   void GetOwnersOfVisibleWindows(
-      std::set<std::string>* user_ids) const override;
+      std::set<user_manager::UserID>* user_ids) const override;
   bool IsWindowOnDesktopOfUser(aura::Window* window,
-                               const std::string& user_id) const override;
-  const std::string& GetUserPresentingWindow(
+                               const user_manager::UserID& user_id) const override;
+  const user_manager::UserID& GetUserPresentingWindow(
       aura::Window* window) const override;
   void AddUser(content::BrowserContext* context) override;
   void AddObserver(Observer* observer) override;
   void RemoveObserver(Observer* observer) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MultiUserWindowManagerStub);
 };
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_WINDOW_MANAGER_STUB_H_