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

chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.h

Index: chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.h
diff --git a/chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.h b/chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.h
index 7a7575c1977487ba4b9d357a6d2bf2e7e0db32b3..deb120682b5cbb6a246fafc882a5965b648b7991 100644
--- a/chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.h
+++ b/chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.h
@@ -20,6 +20,10 @@ namespace policy {
 class ConsumerManagementService;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class SigninScreenHandler;
@@ -146,10 +150,10 @@ class GaiaScreenHandler : public BaseScreenHandler {
                        bool using_saml);
 
   // Fill GAIA user name.
-  void PopulateEmail(const std::string& user_id);
+  void PopulateEmail(const std::string& email);
 
   // Mark user as having password changed:
-  void PasswordChangedFor(const std::string& user_id);
+  void PasswordChangedFor(const user_manager::UserID& user_id);
 
   // Kick off cookie / local storage cleanup.
   void StartClearingCookies(const base::Closure& on_clear_callback);
@@ -160,7 +164,7 @@ class GaiaScreenHandler : public BaseScreenHandler {
   void OnDnsCleared();
 
   // Show sign-in screen for the given credentials.
-  virtual void ShowSigninScreenForCreds(const std::string& username,
+  virtual void ShowSigninScreenForCreds(const user_manager::UserID& user_id,
                                         const std::string& password);
   // Attempts login for test.
   void SubmitLoginFormForTest();
@@ -220,7 +224,7 @@ class GaiaScreenHandler : public BaseScreenHandler {
   std::string populated_email_;
 
   // Emails of the users, whose passwords have recently been changed.
-  std::set<std::string> password_changed_for_;
+  std::set<user_manager::UserID> password_changed_for_;
 
   // True if dns cache cleanup is done.
   bool dns_cleared_;