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

chrome/browser/ui/ash/system_tray_delegate_chromeos.h

Index: chrome/browser/ui/ash/system_tray_delegate_chromeos.h
diff --git a/chrome/browser/ui/ash/system_tray_delegate_chromeos.h b/chrome/browser/ui/ash/system_tray_delegate_chromeos.h
index cbb488291b3e75cd7ddc9f78a4df14dfa4aa6437..2f206641d6d070a425c5bafc394e9b029d609a26 100644
--- a/chrome/browser/ui/ash/system_tray_delegate_chromeos.h
+++ b/chrome/browser/ui/ash/system_tray_delegate_chromeos.h
@@ -45,6 +45,7 @@ class VPNDelegate;
 
 namespace user_manager {
 class User;
+class UserID;
 }
 
 namespace chromeos {
@@ -136,7 +137,7 @@ class SystemTrayDelegateChromeOS
   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(
@@ -254,8 +255,8 @@ class SystemTrayDelegateChromeOS
   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;
@@ -298,7 +299,7 @@ class SystemTrayDelegateChromeOS
   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_;