Auto merge of #14653 - upsuper:bug1321176, r=Manishearth

stylo: Fix assertion for unresolvable url

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This is the Servo part of [bug 1321176](https://bugzilla.mozilla.org/show_bug.cgi?id=1321176), which has been reviewed by @emilio, @Manishearth, and @heycam.

r? @Manishearth

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components/script/dom/element.rs

@@ -397,7 +397,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
                 PropertyDeclaration::BackgroundImage(DeclaredValue::Value(
                     background_image::SpecifiedValue(vec![
                         background_image::single_value::SpecifiedValue(Some(
-                            specified::Image::for_cascade(Some(url.into()), specified::url::UrlExtraData { })
+                            specified::Image::for_cascade(url.into(), specified::url::UrlExtraData { })
                         ))
                     ])))));
         }