Similar to why I'm doing #9172, I'm changing `Rc<RefCell<Response>>` into `Rc<Response>` and wrapping fields in `RefCell<T>` as needed. So far I've only needed to change `url_list`, but I'm sure I'll need to update more fields in the future. There's two lines that don't compile that I'm not going to bother trying to fix until #9172 is approved, because getting them to work now will just require more changes after merging with the changes #9172 makes to them. Source-Repo: https://github.com/servo/servo Source-Revision: b8186a2203cdbdcc24d77554cb53e048ef13c669