Don't resolve URLs at parse time for Stylo.

It's a bit unfortunate the use separate implementations of SpecifiedUrl for Servo
and Gecko, but they're different enough at this point that I don't think it really
makes sense to try to share everything. Splitting them out has some nice
simplifications as well.

I recognize that there's still some potential correctness issues for Servo using
the resolved URI in various places where the original URI may be the right thing,
but I've got too much on my plate to look into that for now.

MozReview-Commit-ID: BeDu93TQ4Ow

components/script/dom/element.rs

@@ -429,7 +429,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
                 PropertyDeclaration::BackgroundImage(
                     background_image::SpecifiedValue(vec![
                         background_image::single_value::SpecifiedValue(Some(
-                            specified::Image::for_cascade(url.into(), specified::url::UrlExtraData { })
+                            specified::Image::for_cascade(url.into())
                         ))
                     ]))));
         }