Add function which convert from URLValueData to SpecifiedUrl.

We will need to convert from URLValueData to SpecifiedUrl when cloning the
stylo's data from gecko's data. Because the filter structure on gecko has url as
URLValue type. We will use this convert function when interpolating as
discrete also(Bug 1368610).

components/style/gecko/url.rs

@@ -5,6 +5,7 @@
 //! Common handling for the specified value CSS url() values.
 
 use gecko_bindings::structs::{ServoBundledURI, URLExtraData};
+use gecko_bindings::structs::mozilla::css::URLValueData;
 use gecko_bindings::structs::root::mozilla::css::ImageValue;
 use gecko_bindings::sugar::refptr::RefPtr;
 use parser::ParserContext;
@@ -51,6 +52,16 @@ impl SpecifiedUrl {
         false
     }
 
+    /// Convert from URLValueData to SpecifiedUrl.
+    pub unsafe fn from_url_value_data(url: &URLValueData)
+                                       -> Result<SpecifiedUrl, ()> {
+        Ok(SpecifiedUrl {
+            serialization: Arc::new(url.mString.to_string()),
+            extra_data: url.mExtraData.to_safe(),
+            image_value: None,
+        })
+    }
+
     /// Returns true if this URL looks like a fragment.
     /// See https://drafts.csswg.org/css-values/#local-urls
     pub fn is_fragment(&self) -> bool {