Remove the url! plugin.

In rust-url 1.0 the `Url` struct is going to have private fields, and there
is no way to to create an aribitrary one without going through the parser.

The plugin never had a clear demonstrated performance benefit,
it was made mostly because it was possible and relatively easy at the time.

components/script/dom/document.rs

@@ -1603,7 +1603,7 @@ impl Document {
                          source: DocumentSource,
                          doc_loader: DocumentLoader)
                          -> Document {
-        let url = url.unwrap_or_else(|| url!("about:blank"));
+        let url = url.unwrap_or_else(|| Url::parse("about:blank").unwrap());
 
         let (ready_state, domcontentloaded_dispatched) = if source == DocumentSource::FromParser {
             (DocumentReadyState::Loading, false)

components/script/dom/htmliframeelement.rs

@@ -146,7 +146,7 @@ impl HTMLIFrameElement {
     pub fn process_the_iframe_attributes(&self) {
         let url = match self.get_url() {
             Some(url) => url.clone(),
-            None => url!("about:blank"),
+            None => Url::parse("about:blank").unwrap(),
         };
 
         self.navigate_or_reload_child_browsing_context(Some(url));

components/script/script_thread.rs

@@ -1880,7 +1880,7 @@ impl ScriptThread {
         };
 
         if load_data.url.scheme == "javascript" {
-            load_data.url = url!("about:blank");
+            load_data.url = Url::parse("about:blank").unwrap();
         }
 
         resource_thread.send(ControlMsg::Load(NetLoadData {