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

chromeos/login/auth/online_attempt_host.h

Index: chromeos/login/auth/online_attempt_host.h
diff --git a/chromeos/login/auth/online_attempt_host.h b/chromeos/login/auth/online_attempt_host.h
index b835e5bf7801d580812edbd05bfe648bc75ec286..cb26b09a2b29a4f6a42b79c6e15fd6ad27c13d20 100644
--- a/chromeos/login/auth/online_attempt_host.h
+++ b/chromeos/login/auth/online_attempt_host.h
@@ -35,7 +35,7 @@ class CHROMEOS_EXPORT OnlineAttemptHost : public AuthAttemptStateResolver {
   class Delegate {
    public:
     // Called after user_context were checked online.
-    virtual void OnChecked(const std::string& username, bool success) = 0;
+    virtual void OnChecked(const user_manager::UserID& user_id, bool success) = 0;
   };
 
   explicit OnlineAttemptHost(Delegate* delegate);