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

chrome/browser/chromeos/login/session/user_session_manager.h

Index: chrome/browser/chromeos/login/session/user_session_manager.h
diff --git a/chrome/browser/chromeos/login/session/user_session_manager.h b/chrome/browser/chromeos/login/session/user_session_manager.h
index b1e74661390cfcaaf651dd4ce3df9ea9e72a6b7d..29671d0d4d6340fba88c715b757e62596faab876 100644
--- a/chrome/browser/chromeos/login/session/user_session_manager.h
+++ b/chrome/browser/chromeos/login/session/user_session_manager.h
@@ -35,6 +35,7 @@ class URLRequestContextGetter;
 
 namespace user_manager {
 class User;
+class UserID;
 }  // namespace user_manager
 
 namespace chromeos {
@@ -247,7 +248,7 @@ class UserSessionManager
   friend class test::UserSessionManagerTestApi;
   friend struct DefaultSingletonTraits<UserSessionManager>;
 
-  typedef std::set<std::string> SigninSessionRestoreStateSet;
+  typedef std::set<user_manager::UserID> SigninSessionRestoreStateSet;
 
   UserSessionManager();
   ~UserSessionManager() override;
@@ -299,7 +300,7 @@ class UserSessionManager
   // Profile is created, extensions and promo resources are initialized.
   void UserProfileInitialized(Profile* profile,
                               bool is_incognito_profile,
-                              const std::string& user_id);
+                              const user_manager::UserID& user_id);
 
   // Callback to resume profile creation after transferring auth data from
   // the authentication profile.
@@ -350,7 +351,7 @@ class UserSessionManager
   void AttemptRestart(Profile* profile);
 
   // Callback invoked when Easy unlock key operations are finished.
-  void OnEasyUnlockKeyOpsFinished(const std::string& user_id,
+  void OnEasyUnlockKeyOpsFinished(const user_manager::UserID& user_id,
                                   bool success);
 
   // Internal implementation of DoBrowserLaunch. Initially should be called with