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

chrome/browser/ui/ash/launcher/multi_profile_app_window_launcher_controller.h

 // 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.
 
 #ifndef CHROME_BROWSER_UI_ASH_LAUNCHER_MULTI_PROFILE_APP_WINDOW_LAUNCHER_CONTROLLER_H_
 #define CHROME_BROWSER_UI_ASH_LAUNCHER_MULTI_PROFILE_APP_WINDOW_LAUNCHER_CONTROLLER_H_
 
 #include "chrome/browser/ui/ash/launcher/app_window_launcher_controller.h"
 
+namespace user_manager {
+class UserID;
+}
+
 // Inherits from AppWindowLauncherController and overwrites the AppWindow
 // observing functions to switch between users dynamically.
 class MultiProfileAppWindowLauncherController
     : public AppWindowLauncherController {
  public:
   explicit MultiProfileAppWindowLauncherController(
       ChromeLauncherController* owner);
   ~MultiProfileAppWindowLauncherController() override;
 
   // Overridden from AppWindowLauncherController:
-  void ActiveUserChanged(const std::string& user_email) override;
+  void ActiveUserChanged(const user_manager::UserID& user_id) override;
   void AdditionalUserAddedToSession(Profile* profile) override;
 
   // Overridden from AppWindowRegistry::Observer:
   void OnAppWindowAdded(extensions::AppWindow* app_window) override;
   void OnAppWindowRemoved(extensions::AppWindow* app_window) override;
   void OnAppWindowShown(extensions::AppWindow* app_window,
                         bool was_hidden) override;
   void OnAppWindowHidden(extensions::AppWindow* app_window) override;
 
  private:
   typedef std::vector<extensions::AppWindow*> AppWindowList;
   typedef std::vector<extensions::AppWindowRegistry*> AppWindowRegistryList;
 
   // Returns true if the owner of the given |app_window| has a window teleported
   // of the |app_window|'s application type to the current desktop.
   bool UserHasAppOnActiveDesktop(extensions::AppWindow* app_window);
 
   // A list of all app windows for all users.
   AppWindowList app_window_list_;
 
   // A list of the app window registries which we additionally observe.
   AppWindowRegistryList multi_user_registry_;
 
   DISALLOW_COPY_AND_ASSIGN(MultiProfileAppWindowLauncherController);
 };
 
 #endif  // CHROME_BROWSER_UI_ASH_LAUNCHER_MULTI_PROFILE_APP_WINDOW_LAUNCHER_CONTROLLER_H_