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

chrome/browser/ui/ash/system_tray_delegate_chromeos.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_SYSTEM_TRAY_DELEGATE_CHROMEOS_H_
 #define CHROME_BROWSER_UI_ASH_SYSTEM_TRAY_DELEGATE_CHROMEOS_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 27 matching lines @@
 #include "extensions/browser/app_window/app_window_registry.h"
 #include "ui/base/ime/chromeos/input_method_manager.h"
 #include "ui/chromeos/ime/input_method_menu_manager.h"
 
 namespace ash {
 class VPNDelegate;
 }
 
 namespace user_manager {
 class User;
+class UserID;
 }
 
 namespace chromeos {
 
 class SystemTrayDelegateChromeOS
     : public ui::ime::InputMethodMenuManager::Observer,
       public ash::SystemTrayDelegate,
       public SessionManagerClient::Observer,
       public content::NotificationObserver,
       public input_method::InputMethodManager::Observer,
@@ skipping 71 matching lines @@
   ash::NetworkingConfigDelegate* GetNetworkingConfigDelegate() const override;
   ash::VolumeControlDelegate* GetVolumeControlDelegate() const override;
   void SetVolumeControlDelegate(
       scoped_ptr<ash::VolumeControlDelegate> delegate) override;
   bool GetSessionStartTime(base::TimeTicks* session_start_time) override;
   bool GetSessionLengthLimit(base::TimeDelta* session_length_limit) override;
   int GetSystemTrayMenuWidth() override;
   void ActiveUserWasChanged() override;
   bool IsSearchKeyMappedToCapsLock() override;
   ash::tray::UserAccountsDelegate* GetUserAccountsDelegate(
-      const std::string& user_id) override;
+      const user_manager::UserID& user_id) override;
   void AddCustodianInfoTrayObserver(
       ash::CustodianInfoTrayObserver* observer) override;
   void RemoveCustodianInfoTrayObserver(
       ash::CustodianInfoTrayObserver* observer) override;
   void AddShutdownPolicyObserver(
       ash::ShutdownPolicyObserver* observer) override;
   void RemoveShutdownPolicyObserver(
       ash::ShutdownPolicyObserver* observer) override;
   void ShouldRebootOnShutdown(
       const ash::RebootOnShutdownCallback& callback) override;
@@ skipping 97 matching lines @@
   void OnStartBluetoothDiscoverySession(
       scoped_ptr<device::BluetoothDiscoverySession> discovery_session);
 
   void UpdateEnterpriseDomain();
 
   // Overridden from CloudPolicyStore::Observer
   void OnStoreLoaded(policy::CloudPolicyStore* store) override;
   void OnStoreError(policy::CloudPolicyStore* store) override;
 
   // Overridden from ash::SessionStateObserver
-  void UserAddedToSession(const std::string& user_id) override;
-  void ActiveUserChanged(const std::string& user_id) override;
+  void UserAddedToSession(const user_manager::UserID& user_id) override;
+  void ActiveUserChanged(const user_manager::UserID& user_id) override;
 
   // Overridden from chrome::BrowserListObserver:
   void OnBrowserRemoved(Browser* browser) override;
 
   // Overridden from extensions::AppWindowRegistry::Observer:
   void OnAppWindowRemoved(extensions::AppWindow* app_window) override;
 
   // Overridden from SupervisedUserServiceObserver:
   void OnCustodianInfoChanged() override;
 
@@ skipping 22 matching lines @@
   bool should_run_bluetooth_discovery_;
   bool session_started_;
 
   scoped_refptr<device::BluetoothAdapter> bluetooth_adapter_;
   scoped_ptr<device::BluetoothDiscoverySession> bluetooth_discovery_session_;
   scoped_ptr<ash::CastConfigDelegate> cast_config_delegate_;
   scoped_ptr<ash::NetworkingConfigDelegate> networking_config_delegate_;
   scoped_ptr<ash::VolumeControlDelegate> volume_control_delegate_;
   scoped_ptr<CrosSettingsObserverSubscription> device_settings_observer_;
   scoped_ptr<AccessibilityStatusSubscription> accessibility_subscription_;
-  base::ScopedPtrHashMap<std::string,
+  base::ScopedPtrHashMap<user_manager::UserID,
                          scoped_ptr<ash::tray::UserAccountsDelegate>>
       accounts_delegates_;
   scoped_ptr<ShutdownPolicyHandler> shutdown_policy_handler_;
   scoped_ptr<ash::VPNDelegate> vpn_delegate_;
 
   ObserverList<ash::CustodianInfoTrayObserver>
       custodian_info_changed_observers_;
 
   ObserverList<ash::ShutdownPolicyObserver> shutdown_policy_observers_;
 
   base::WeakPtrFactory<SystemTrayDelegateChromeOS> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SystemTrayDelegateChromeOS);
 };
 
 ash::SystemTrayDelegate* CreateSystemTrayDelegate();
 
 }  // namespace chromeos
 #endif  // CHROME_BROWSER_UI_ASH_SYSTEM_TRAY_DELEGATE_CHROMEOS_H_