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

chrome/browser/chromeos/login/supervised/supervised_user_creation_flow.h

 // Copyright 2014 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_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_CREATION_FLOW_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_CREATION_FLOW_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/strings/string16.h"
 #include "chrome/browser/chromeos/login/user_flow.h"
 #include "components/user_manager/user.h"
 
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 // UserFlow implementation for creating new supervised user.
 class SupervisedUserCreationFlow : public ExtendedUserFlow {
  public:
-  explicit SupervisedUserCreationFlow(const std::string& manager_id);
+  explicit SupervisedUserCreationFlow(const user_manager::UserID& manager_id);
   ~SupervisedUserCreationFlow() override;
 
   bool CanLockScreen() override;
   bool ShouldShowSettings() override;
   bool ShouldLaunchBrowser() override;
   bool ShouldSkipPostLoginScreens() override;
   bool SupportsEarlyRestartToApplyFlags() override;
   bool HandleLoginFailure(const AuthFailure& failure) override;
   void HandleLoginSuccess(const UserContext& context) override;
   bool HandlePasswordChangeDetected() override;
@@ skipping 19 matching lines @@
   bool session_started_;
 
   Profile* manager_profile_;
 
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserCreationFlow);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_CREATION_FLOW_H_