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

chromeos/login/auth/authenticator.h

Index: chromeos/login/auth/authenticator.h
diff --git a/chromeos/login/auth/authenticator.h b/chromeos/login/auth/authenticator.h
index 650ea382aea0f2cf00f1ab427e0affe0e6f4fc89..b22ed1713a0b3745f18e9b8305541983dc9e85cf 100644
--- a/chromeos/login/auth/authenticator.h
+++ b/chromeos/login/auth/authenticator.h
@@ -17,6 +17,10 @@ namespace content {
 class BrowserContext;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class UserContext;
@@ -60,7 +64,7 @@ class CHROMEOS_EXPORT Authenticator
   // The |app_user_id| is a generated username for the account.
   // |use_guest_mount| specifies whether to force the session to use a
   // guest mount. If this is false, we use mount a public cryptohome.
-  virtual void LoginAsKioskAccount(const std::string& app_user_id,
+  virtual void LoginAsKioskAccount(const user_manager::UserID& app_user_id,
                                    bool use_guest_mount) = 0;
 
   // Notifies caller that login was successful. Must be called on the UI thread.