Auto merge of #12648 - heycam:store-extra-data, r=Manishearth

Store UrlExtraData in {specified,computed}::Image::Url.

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This stores a `UrlExtraData` object in `specified::Image::Url` and `computed::Image::Url`, so that geckolib can use the base/principal to create Gecko `ImageValue`s.  (I'll do that in the followup to support background-image; want to get this in first.)

r? @Manishearth

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- [ ] These changes fix #__ (github issue number if applicable).

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components/script/dom/element.rs

@@ -347,7 +347,8 @@ impl LayoutElementHelpers for LayoutJS<Element> {
         if let Some(url) = background {
             hints.push(from_declaration(
                 PropertyDeclaration::BackgroundImage(DeclaredValue::Value(
-                    background_image::SpecifiedValue(Some(specified::Image::Url(url)))))));
+                    background_image::SpecifiedValue(Some(
+                        specified::Image::Url(url, specified::UrlExtraData { })))))));
         }
 
         let color = if let Some(this) = self.downcast::<HTMLFontElement>() {