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

chrome/browser/chromeos/login/ui/login_display.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_UI_LOGIN_DISPLAY_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_UI_LOGIN_DISPLAY_H_
 
 #include <string>
 #include <vector>
 
 #include "base/callback.h"
 #include "base/strings/string16.h"
 #include "chrome/browser/chromeos/login/help_app_launcher.h"
 #include "chrome/browser/chromeos/login/signin_specifics.h"
 #include "components/user_manager/user.h"
 #include "components/user_manager/user_manager.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/image/image.h"
 #include "ui/gfx/native_widget_types.h"
 
+namespace user_manager {
+class UserID;
+} // namespace user_manager
+
 namespace chromeos {
 
 class UserContext;
 
 // TODO(nkostylev): Extract interface, create a BaseLoginDisplay class.
 // An abstract class that defines login UI implementation.
 class LoginDisplay {
  public:
   // Sign in error IDs that require detailed error screen and not just
   // a simple error bubble.
@@ skipping 91 matching lines @@
   // |login_attempts| shows number of login attempts made by current user.
   // |help_topic_id| is additional help topic that is presented as link.
   virtual void ShowError(int error_msg_id,
                          int login_attempts,
                          HelpAppLauncher::HelpTopic help_topic_id) = 0;
 
   // Displays detailed error screen for error with ID |error_id|.
   virtual void ShowErrorScreen(LoginDisplay::SigninError error_id) = 0;
 
   // Proceed with Gaia flow because password has changed.
-  virtual void ShowGaiaPasswordChanged(const std::string& username) = 0;
+  virtual void ShowGaiaPasswordChanged(const user_manager::UserID& user_id) = 0;
 
   // Show password changed dialog. If |show_password_error| is not null
   // user already tried to enter old password but it turned out to be incorrect.
   virtual void ShowPasswordChangedDialog(bool show_password_error,
-                                         const std::string& email) = 0;
+                                         const user_manager::UserID& user_id) = 0;
 
-  // Shows signin UI with specified email.
-  virtual void ShowSigninUI(const std::string& email) = 0;
+  // Shows signin UI with specified UserID.
+  virtual void ShowSigninUI(const user_manager::UserID& user_id) = 0;
 
   // Show whitelist check failed error. Happens after user completes online
   // signin but whitelist check fails.
   virtual void ShowWhitelistCheckFailedError() = 0;
 
   gfx::Rect background_bounds() const { return background_bounds_; }
   void set_background_bounds(const gfx::Rect& background_bounds) {
     background_bounds_ = background_bounds;
   }
 
@@ skipping 23 matching lines @@
   // in redesigned login stack.
   // Login stack (and this object) will be recreated for next user sign in.
   bool is_signin_completed_;
 
   DISALLOW_COPY_AND_ASSIGN(LoginDisplay);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_UI_LOGIN_DISPLAY_H_