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

chrome/browser/chromeos/login/easy_unlock/easy_unlock_user_login_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_EASY_UNLOCK_EASY_UNLOCK_USER_LOGIN_FLOW_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_EASY_UNLOCK_EASY_UNLOCK_USER_LOGIN_FLOW_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "chrome/browser/chromeos/login/user_flow.h"
 
+namespace user_manager {
+class UserID;
+}
+
 // Handler for login flow initiazted by Easy Signin login attempt.
 // The only difference to the default login flow is hanlding of the auth
 // failure.
 class EasyUnlockUserLoginFlow : public chromeos::ExtendedUserFlow {
  public:
-  explicit EasyUnlockUserLoginFlow(const std::string& user_id);
+  explicit EasyUnlockUserLoginFlow(const user_manager::UserID& user_id);
   ~EasyUnlockUserLoginFlow() override;
 
  private:
   // chromeos::ExtendedUserFlow implementation.
   bool CanLockScreen() override;
   bool ShouldLaunchBrowser() override;
   bool ShouldSkipPostLoginScreens() override;
   bool HandleLoginFailure(const chromeos::AuthFailure& failure) override;
   void HandleLoginSuccess(const chromeos::UserContext& context) override;
   bool HandlePasswordChangeDetected() override;
   void HandleOAuthTokenStatusChange(
       user_manager::User::OAuthTokenStatus status) override;
   void LaunchExtraSteps(Profile* profile) override;
   bool SupportsEarlyRestartToApplyFlags() override;
 
   DISALLOW_COPY_AND_ASSIGN(EasyUnlockUserLoginFlow);
 };
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_EASY_UNLOCK_EASY_UNLOCK_USER_LOGIN_FLOW_H_