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

chrome/browser/chromeos/login/ui/login_display.h

Index: chrome/browser/chromeos/login/ui/login_display.h
diff --git a/chrome/browser/chromeos/login/ui/login_display.h b/chrome/browser/chromeos/login/ui/login_display.h
index 6512bcf854c38168b41e728a48c9403f5b9d5e04..c2ef1b245d4691018b5814275e688160d8797482 100644
--- a/chrome/browser/chromeos/login/ui/login_display.h
+++ b/chrome/browser/chromeos/login/ui/login_display.h
@@ -18,6 +18,10 @@
 #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;
@@ -129,15 +133,15 @@ class LoginDisplay {
   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.