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

chrome/browser/chromeos/app_mode/kiosk_profile_loader.h

Index: chrome/browser/chromeos/app_mode/kiosk_profile_loader.h
diff --git a/chrome/browser/chromeos/app_mode/kiosk_profile_loader.h b/chrome/browser/chromeos/app_mode/kiosk_profile_loader.h
index 42796e398b3d05e9809f51759d6dfa3cceb2c5a8..46f85d602789f9818ae064530ee04951d6bcf73d 100644
--- a/chrome/browser/chromeos/app_mode/kiosk_profile_loader.h
+++ b/chrome/browser/chromeos/app_mode/kiosk_profile_loader.h
@@ -33,7 +33,7 @@ class KioskProfileLoader : public LoginPerformer::Delegate,
     virtual ~Delegate() {}
   };
 
-  KioskProfileLoader(const std::string& app_user_id,
+  KioskProfileLoader(const user_manager::UserID& app_user_id,
                      bool use_guest_mount,
                      Delegate* delegate);
 
@@ -51,14 +51,14 @@ class KioskProfileLoader : public LoginPerformer::Delegate,
   // LoginPerformer::Delegate overrides:
   void OnAuthSuccess(const UserContext& user_context) override;
   void OnAuthFailure(const AuthFailure& error) override;
-  void WhiteListCheckFailed(const std::string& email) override;
+  void WhiteListCheckFailed(const user_manager::UserID& user_id) override;
   void PolicyLoadFailed() override;
-  void OnOnlineChecked(const std::string& email, bool success) override;
+  void OnOnlineChecked(const user_manager::UserID& user_id, bool success) override;
 
   // UserSessionManagerDelegate implementation:
   void OnProfilePrepared(Profile* profile, bool browser_launched) override;
 
-  std::string user_id_;
+  user_manager::UserID user_id_;
   bool use_guest_mount_;
   Delegate* delegate_;
   scoped_ptr<CryptohomedChecker> cryptohomed_checker_;