style: Add a StylesheetLoader abstraction, and make it a no-op on Geckolib.

Servo doesn't compile at this stage.

components/layout_thread/lib.rs

@@ -1542,6 +1542,7 @@ fn get_ua_stylesheets() -> Result<UserAgentStylesheets, &'static str> {
             None,
             Origin::UserAgent,
             Default::default(),
+            None,
             Box::new(StdoutErrorReporter),
             ParserContextExtraData::default()))
     }
@@ -1555,7 +1556,8 @@ fn get_ua_stylesheets() -> Result<UserAgentStylesheets, &'static str> {
     for &(ref contents, ref url) in &opts::get().user_stylesheets {
         user_or_user_agent_stylesheets.push(Stylesheet::from_bytes(
             &contents, url.clone(), None, None, Origin::User, Default::default(),
-            Box::new(StdoutErrorReporter), ParserContextExtraData::default()));
+            None, Box::new(StdoutErrorReporter),
+            ParserContextExtraData::default()));
     }
 
     let quirks_mode_stylesheet = try!(parse_ua_stylesheet("quirks-mode.css"));

components/style/stylesheets.rs

@@ -307,6 +307,7 @@ impl CssRule {
         let mut rule_parser = TopLevelRuleParser {
             stylesheet_origin: parent_stylesheet.origin,
             context: context,
+            loader: None,
             state: Cell::new(state),
             namespaces: &mut namespaces,
         };
@@ -605,9 +606,20 @@ rule_filter! {
     effective_keyframes_rules(Keyframes => KeyframesRule),
 }
 
+/// The stylesheet loader is the abstraction used to trigger network requests
+/// for `@import` rules.
+pub trait StylesheetLoader {
+    /// Request a stylesheet after parsing a given `@import` rule.
+    ///
+    /// The called code is responsible to update the `stylesheet` rules field
+    /// when the sheet is done loading.
+    fn request_stylesheet(&self, import: &Arc<RwLock<ImportRule>>);
+}
+
 struct TopLevelRuleParser<'a> {
     stylesheet_origin: Origin,
     namespaces: &'a mut Namespaces,
+    loader: Option<&'a StylesheetLoader>,
     context: ParserContext<'a>,
     state: Cell<State>,
 }
@@ -679,6 +691,12 @@ impl<'a> AtRuleParser for TopLevelRuleParser<'a> {
                         }
                     ));
 
+                    if is_valid_url {
+                        let loader = self.loader
+                            .expect("Expected a stylesheet loader for @import");
+                        loader.request_stylesheet(&import_rule);
+                    }
+
                     return Ok(AtRuleType::WithoutBlock(CssRule::Import(import_rule)))
                 } else {
                     self.state.set(State::Invalid);