style: Change URL value storage in StyleShapeSource.

This is due to Bug 1404243 Part 4.

MozReview-Commit-ID: CFRCdw2Dayy

components/style/gecko/conversions.rs

@@ -617,7 +617,8 @@ pub mod basic_shape {
                 StyleShapeSourceType::Box => ShapeSource::Box(other.mReferenceBox.into()),
                 StyleShapeSourceType::URL => {
                     unsafe {
-                        let other_url = &(**other.__bindgen_anon_1.mURL.as_ref());
+                        let shape_image = &*other.mShapeImage.mPtr;
+                        let other_url = &(**shape_image.__bindgen_anon_1.mURLValue.as_ref());
                         let url = SpecifiedUrl::from_url_value_data(&other_url._base).unwrap();
                         ShapeSource::Url(url)
                     }