Associate ResourceTask with UrlProvenance

Fixes #2121

I thought this would be better than passing an `Option<ResourceTask>` to `parse_css` because it avoids having to handle the potential failure at runtime when unwrapping.

No test included, but the error was reproducible by running content/test_getBoundingClientRect.html, and this fixes it. Glad to find a way to add an explicit test if need be.