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

chrome/browser/ui/ash/launcher/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_APP_WINDOW_LAUNCHER_CONTROLLER_H_
 #define CHROME_BROWSER_UI_ASH_LAUNCHER_APP_WINDOW_LAUNCHER_CONTROLLER_H_
 
 #include <list>
 #include <map>
 #include <string>
 
 #include "extensions/browser/app_window/app_window_registry.h"
 #include "ui/aura/window_observer.h"
 #include "ui/wm/public/activation_change_observer.h"
 
 namespace aura {
 
 class Window;
 
 namespace client {
 class ActivationClient;
 }
 }
 
 namespace extensions {
 class AppWindow;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 class ChromeLauncherController;
 class Profile;
 class AppWindowLauncherItemController;
 
 // AppWindowLauncherController observes the app window registry and the
 // aura window manager. It handles adding and removing launcher items from
 // ChromeLauncherController.
 class AppWindowLauncherController
     : public extensions::AppWindowRegistry::Observer,
       public aura::WindowObserver,
       public aura::client::ActivationChangeObserver {
  public:
   explicit AppWindowLauncherController(ChromeLauncherController* owner);
   ~AppWindowLauncherController() override;
 
   // Called by ChromeLauncherController when the active user changed and the
   // items need to be updated.
-  virtual void ActiveUserChanged(const std::string& user_email) {}
+  virtual void ActiveUserChanged(const user_manager::UserID& user_email) {}
 
   // An additional user identified by |Profile|, got added to the existing
   // session.
   virtual void AdditionalUserAddedToSession(Profile* profile);
 
   // Overridden from AppWindowRegistry::Observer:
   void OnAppWindowIconChanged(extensions::AppWindow* app_window) override;
   void OnAppWindowShown(extensions::AppWindow* app_window,
                         bool was_hidden) override;
   void OnAppWindowHidden(extensions::AppWindow* app_window) override;
@@ skipping 32 matching lines @@
   // Map of app launcher id to controller.
   AppControllerMap app_controller_map_;
 
   // Allows us to get from an aura::Window to the app shelf id.
   WindowToAppShelfIdMap window_to_app_shelf_id_map_;
 
   DISALLOW_COPY_AND_ASSIGN(AppWindowLauncherController);
 };
 
 #endif  // CHROME_BROWSER_UI_ASH_LAUNCHER_APP_WINDOW_LAUNCHER_CONTROLLER_H_