Investigate eliminating Arc from `CssUrlData::original`

[Loirooriol]

From Emilio's review in https://phabricator.services.mozilla.com/D261582#inline-1434189

Do you need this Arc now? Probably not?

stylo/style/servo/url.rs

Line 43 in 8d42ac8

original: Option<Arc<String>>,

[nicoburns]

I think the suggestion is to make it Option<String> (because the containing struct is now already wrapped in Arc) rather than removing the field entirely (although that would be great if we could).