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

components/user_manager/remove_user_delegate.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 COMPONENTS_USER_MANAGER_REMOVE_USER_DELEGATE_H_
 #define COMPONENTS_USER_MANAGER_REMOVE_USER_DELEGATE_H_
 
 #include "components/user_manager/user_manager_export.h"
 
 namespace user_manager {
 
 // Delegate to be used with |UserManager::RemoveUser|.
 class USER_MANAGER_EXPORT RemoveUserDelegate {
  public:
   // Called right before actual user removal process is initiated.
-  virtual void OnBeforeUserRemoved(const std::string& username) = 0;
+  virtual void OnBeforeUserRemoved(const UserID& user_id) = 0;
 
   // Called right after user removal process has been initiated.
-  virtual void OnUserRemoved(const std::string& username) = 0;
+  virtual void OnUserRemoved(const UserID& user_id) = 0;
 };
 
 }  // namespace user_manager
 
 #endif  // COMPONENTS_USER_MANAGER_REMOVE_USER_DELEGATE_H_