We now don't absolutize the URL during parsing, keeping it as a CSS::URL
object which we then pass to the "fetch an external image for a
stylesheet" algorithm. Our version of this algorithm is a bit ad-hoc,
in order to make use of SharedResourceRequest. To try and reduce
duplication, I've pulled all of fetch_a_style_resource() into a static
function, apart from the "actually do the fetch" step.
See the spec issue in the comments for details. There are situations
where we will need to call this but don't have a CSSStyleSheet to pass
in, but we always have a Document, so use that where possible.
