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

chrome/browser/chromeos/login/existing_user_controller.h

Index: chrome/browser/chromeos/login/existing_user_controller.h
diff --git a/chrome/browser/chromeos/login/existing_user_controller.h b/chrome/browser/chromeos/login/existing_user_controller.h
index 0eb01b89eade16b911f5c5776b7896a3cdd45e4e..4cc9a144d391e0ed4adffeb19741597701e9887e 100644
--- a/chrome/browser/chromeos/login/existing_user_controller.h
+++ b/chrome/browser/chromeos/login/existing_user_controller.h
@@ -24,6 +24,7 @@
 #include "chromeos/login/auth/login_performer.h"
 #include "chromeos/login/auth/user_context.h"
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "ui/gfx/geometry/rect.h"
@@ -132,7 +133,7 @@ class ExistingUserController : public LoginDisplay::Delegate,
 
   void LoginAsGuest();
   void LoginAsPublicSession(const UserContext& user_context);
-  void LoginAsKioskApp(const std::string& app_id, bool diagnostic_mode);
+  void LoginAsKioskApp(const user_manager::UserID& user_id, bool diagnostic_mode);
 
   // Retrieve public session auto-login policy and update the timer.
   void ConfigurePublicSessionAutoLogin();
@@ -145,9 +146,9 @@ class ExistingUserController : public LoginDisplay::Delegate,
   void OnAuthSuccess(const UserContext& user_context) override;
   void OnOffTheRecordAuthSuccess() override;
   void OnPasswordChangeDetected() override;
-  void WhiteListCheckFailed(const std::string& email) override;
+  void WhiteListCheckFailed(const user_manager::UserID& user_id) override;
   void PolicyLoadFailed() override;
-  void OnOnlineChecked(const std::string& username, bool success) override;
+  void OnOnlineChecked(const user_manager::UserID& user_id, bool success) override;
 
   // UserSessionManagerDelegate implementation:
   void OnProfilePrepared(Profile* profile, bool browser_launched) override;
@@ -164,7 +165,7 @@ class ExistingUserController : public LoginDisplay::Delegate,
   void ShowError(int error_id, const std::string& details);
 
   // Shows Gaia page because password change was detected.
-  void ShowGaiaPasswordChanged(const std::string& username);
+  void ShowGaiaPasswordChanged(const user_manager::UserID& user_id);
 
   // Handles result of ownership check and starts enterprise or kiosk enrollment
   // if applicable.
@@ -248,8 +249,8 @@ class ExistingUserController : public LoginDisplay::Delegate,
   // Public session auto-login timeout, in milliseconds.
   int public_session_auto_login_delay_;
 
-  // Username for public session auto-login.
-  std::string public_session_auto_login_username_;
+  // User id for public session auto-login.
+  user_manager::UserID public_session_auto_login_user_id_;
 
   // Used to execute login operations.
   scoped_ptr<LoginPerformer> login_performer_;
@@ -259,7 +260,7 @@ class ExistingUserController : public LoginDisplay::Delegate,
   AuthStatusConsumer* auth_status_consumer_;
 
   // Username of the last login attempt.
-  std::string last_login_attempt_username_;
+  user_manager::UserID last_login_attempt_user_id_;
 
   // OOBE/login display host.
   LoginDisplayHost* host_;
@@ -294,7 +295,7 @@ class ExistingUserController : public LoginDisplay::Delegate,
   bool is_login_in_progress_;
 
   // Whether online login attempt succeeded.
-  std::string online_succeeded_for_;
+  user_manager::UserID online_succeeded_for_;
 
   // True if password has been changed for user who is completing sign in.
   // Set in OnLoginSuccess. Before that use LoginPerformer::password_changed().