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

chrome/browser/ui/ash/launcher/browser_status_monitor.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_LAUNCHER_BROWSER_STATUS_MONITOR_H_
 #define CHROME_BROWSER_UI_ASH_LAUNCHER_BROWSER_STATUS_MONITOR_H_
 
 #include <map>
 #include <string>
 
 #include "ash/shelf/scoped_observer_with_duplicated_sources.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/scoped_observer.h"
 #include "chrome/browser/ui/ash/launcher/chrome_launcher_controller.h"
 #include "chrome/browser/ui/browser_list_observer.h"
 #include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
 #include "ui/aura/window_observer.h"
 #include "ui/gfx/display_observer.h"
 #include "ui/wm/public/activation_change_observer.h"
 
 namespace aura {
 class Window;
 
 namespace client {
 class ActivationClient;
 }
 }  // namespace aura
 
+namespace user_manager {
+class UserID;
+}
+
 class Browser;
 
 // BrowserStatusMonitor monitors creation/deletion of Browser and its
 // TabStripModel to keep the launcher representation up to date as the
 // active tab changes.
 class BrowserStatusMonitor : public aura::client::ActivationChangeObserver,
                              public aura::WindowObserver,
                              public chrome::BrowserListObserver,
                              public gfx::DisplayObserver,
                              public TabStripModelObserver {
  public:
   explicit BrowserStatusMonitor(ChromeLauncherController* launcher_controller);
   ~BrowserStatusMonitor() override;
 
   // A function which gets called when the current user has changed.
   // Note that this function is called by the ChromeLauncherController to be
   // able to do the activation in a proper order - rather then setting an
   // observer.
-  virtual void ActiveUserChanged(const std::string& user_email) {}
+  virtual void ActiveUserChanged(const user_manager::UserID& user_email) {}
 
   // A shortcut to call the ChromeLauncherController's UpdateAppState().
   void UpdateAppItemState(content::WebContents* contents,
                           ChromeLauncherController::AppState app_state);
 
   // A shortcut to call the BrowserShortcutLauncherItemController's
   // UpdateBrowserItemState().
   void UpdateBrowserItemState();
 
   // aura::client::ActivationChangeObserver overrides:
@@ skipping 77 matching lines @@
   ScopedObserver<aura::Window, aura::WindowObserver> observed_root_windows_;
 
   BrowserToAppIDMap browser_to_app_id_map_;
   WebContentsToObserverMap webcontents_to_observer_map_;
   scoped_ptr<SettingsWindowObserver> settings_window_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserStatusMonitor);
 };
 
 #endif  // CHROME_BROWSER_UI_ASH_LAUNCHER_BROWSER_STATUS_MONITOR_H_