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

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

Index: chrome/browser/ui/webui/chromeos/login/supervised_user_creation_screen_handler.h
diff --git a/chrome/browser/ui/webui/chromeos/login/supervised_user_creation_screen_handler.h b/chrome/browser/ui/webui/chromeos/login/supervised_user_creation_screen_handler.h
index 4d9284be8221bcabfbb3aecf94d7031d3cdf379d..47920c8e7931b35450258bdc6b24011144e3b372 100644
--- a/chrome/browser/ui/webui/chromeos/login/supervised_user_creation_screen_handler.h
+++ b/chrome/browser/ui/webui/chromeos/login/supervised_user_creation_screen_handler.h
@@ -17,6 +17,10 @@ namespace base {
 class ListValue;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class SupervisedUserCreationScreenHandler : public BaseScreenHandler {
@@ -32,7 +36,7 @@ class SupervisedUserCreationScreenHandler : public BaseScreenHandler {
 
     // Starts supervised user creation flow, with manager identified by
     // |manager_id| and |manager_password|.
-    virtual void AuthenticateManager(const std::string& manager_id,
+    virtual void AuthenticateManager(const user_manager::UserID& manager_id,
                                      const std::string& manager_password) = 0;
 
     // Starts supervised user creation flow, with supervised user that would
@@ -48,11 +52,11 @@ class SupervisedUserCreationScreenHandler : public BaseScreenHandler {
                                        std::string *out_id) const = 0;
 
     // Starts supervised user import flow for user identified with |user_id|.
-    virtual void ImportSupervisedUser(const std::string& user_id) = 0;
+    virtual void ImportSupervisedUser(const user_manager::UserID& user_id) = 0;
     // Starts supervised user import flow for user identified with |user_id| and
     // additional |password|.
     virtual void ImportSupervisedUserWithPassword(
-        const std::string& user_id, const std::string& password) = 0;
+        const user_manager::UserID& user_id, const std::string& password) = 0;
 
     virtual void AbortFlow() = 0;
     virtual void FinishFlow() = 0;