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

chrome/browser/chromeos/login/easy_unlock/bootstrap_manager.h

 // Copyright 2015 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_EASY_UNLOCK_BOOTSTRAP_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_EASY_UNLOCK_BOOTSTRAP_MANAGER_H_
 
 #include <string>
 
 #include "base/macros.h"
 
 class PrefRegistrySimple;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 // BootstrapManager manages bootstrap user specific values such as tracking
 // pending bootstrap and clearing such state.
 class BootstrapManager {
  public:
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
   class Delegate {
    public:
-    virtual void RemovePendingBootstrapUser(const std::string& user_id) = 0;
+    virtual void RemovePendingBootstrapUser(const user_manager::UserID& user_id) = 0;
 
    protected:
     virtual ~Delegate() {}
   };
 
   explicit BootstrapManager(Delegate* delegate);
   ~BootstrapManager();
 
-  void AddPendingBootstrap(const std::string& user_id);
-  void FinishPendingBootstrap(const std::string& user_id);
+  void AddPendingBootstrap(const user_manager::UserID& user_id);
+  void FinishPendingBootstrap(const user_manager::UserID& user_id);
   void RemoveAllPendingBootstrap();
 
-  bool HasPendingBootstrap(const std::string& user_id) const;
+  bool HasPendingBootstrap(const user_manager::UserID& user_id) const;
 
  private:
   Delegate* delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(BootstrapManager);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_EASY_UNLOCK_BOOTSTRAP_MANAGER_H_