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

components/user_manager/remove_user_delegate.h

Index: components/user_manager/remove_user_delegate.h
diff --git a/components/user_manager/remove_user_delegate.h b/components/user_manager/remove_user_delegate.h
index 1077bda527fa6f11b4c82c43e7dd113d9d125bd8..0b1ab022ec006719c82ba22981f496740a14750c 100644
--- a/components/user_manager/remove_user_delegate.h
+++ b/components/user_manager/remove_user_delegate.h
@@ -13,10 +13,10 @@ namespace user_manager {
 class USER_MANAGER_EXPORT RemoveUserDelegate {
  public:
   // Called right before actual user removal process is initiated.
-  virtual void OnBeforeUserRemoved(const std::string& username) = 0;
+  virtual void OnBeforeUserRemoved(const UserID& user_id) = 0;
 
   // Called right after user removal process has been initiated.
-  virtual void OnUserRemoved(const std::string& username) = 0;
+  virtual void OnUserRemoved(const UserID& user_id) = 0;
 };
 
 }  // namespace user_manager