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

chrome/browser/chromeos/login/user_flow.h

 // Copyright (c) 2012 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_USER_FLOW_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USER_FLOW_H_
 
 #include "base/compiler_specific.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chromeos/login/auth/auth_status_consumer.h"
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 
 namespace chromeos {
 
 class UserContext;
 
 class LoginDisplayHost;
 // Defines possible variants of user flow upon logging in.
 // See UserManager::SetUserFlow for usage contract.
 class UserFlow {
  public:
@@ skipping 40 matching lines @@
   void HandleLoginSuccess(const UserContext& context) override;
   bool HandlePasswordChangeDetected() override;
   void HandleOAuthTokenStatusChange(
       user_manager::User::OAuthTokenStatus status) override;
   void LaunchExtraSteps(Profile* profile) override;
 };
 
 // UserFlow stub for non-regular flows.
 class ExtendedUserFlow : public UserFlow {
  public:
-  explicit ExtendedUserFlow(const std::string& user_id);
+  explicit ExtendedUserFlow(const user_manager::UserID& user_id);
   ~ExtendedUserFlow() override;
 
   void AppendAdditionalCommandLineSwitches() override;
   bool ShouldShowSettings() override;
   void HandleOAuthTokenStatusChange(
       user_manager::User::OAuthTokenStatus status) override;
 
  protected:
   // Subclasses can call this method to unregister flow in the next event.
   virtual void UnregisterFlowSoon();
-  std::string user_id() {
+  user_manager::UserID user_id() {
     return user_id_;
   }
 
  private:
-  std::string user_id_;
+  user_manager::UserID user_id_;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USER_FLOW_H_