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

Servo doesn't compile at this stage.

ports/geckolib/glue.rs

@@ -60,6 +60,7 @@ use style::thread_state;
 use style::timer::Timer;
 use style::traversal::{recalc_style_at, DomTraversalContext, PerLevelTraversalData};
 use style_traits::ToCss;
+use stylesheet_loader::StylesheetLoader;
 
 /*
  * For Gecko->Servo function calls, we need to redeclare the same signature that was declared in
@@ -232,8 +233,10 @@ pub extern "C" fn Servo_StyleSheet_Empty(mode: SheetParsingMode) -> RawServoStyl
         SheetParsingMode::eUserSheetFeatures => Origin::User,
         SheetParsingMode::eAgentSheetFeatures => Origin::UserAgent,
     };
+    let loader = StylesheetLoader::new();
     let sheet = Arc::new(Stylesheet::from_str(
-        "", url, origin, Default::default(), Box::new(StdoutErrorReporter), extra_data));
+        "", url, origin, Default::default(), Some(&loader),
+        Box::new(StdoutErrorReporter), extra_data));
     unsafe {
         transmute(sheet)
     }
@@ -262,8 +265,10 @@ pub extern "C" fn Servo_StyleSheet_FromUTF8Bytes(data: *const nsACString,
         referrer: Some(GeckoArcURI::new(referrer)),
         principal: Some(GeckoArcPrincipal::new(principal)),
     }};
+    let loader = StylesheetLoader::new();
     let sheet = Arc::new(Stylesheet::from_str(
-        input, url, origin, Default::default(), Box::new(StdoutErrorReporter), extra_data));
+        input, url, origin, Default::default(), Some(&loader),
+        Box::new(StdoutErrorReporter), extra_data));
     unsafe {
         transmute(sheet)
     }

ports/geckolib/lib.rs

@@ -18,6 +18,7 @@ extern crate style_traits;
 
 #[allow(non_snake_case)]
 pub mod glue;
+mod stylesheet_loader;
 
 // FIXME(bholley): This should probably go away once we harmonize the allocators.
 #[no_mangle]

ports/geckolib/stylesheet_loader.rs

@@ -0,0 +1,21 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use parking_lot::RwLock;
+use std::sync::Arc;
+use style::stylesheets::{ImportRule, StylesheetLoader as StyleStylesheetLoader};
+
+pub struct StylesheetLoader;
+
+impl StylesheetLoader {
+    pub fn new() -> Self {
+        StylesheetLoader
+    }
+}
+
+impl StyleStylesheetLoader for StylesheetLoader {
+    fn request_stylesheet(&self, _import: &Arc<RwLock<ImportRule>>) {
+        // FIXME(emilio): Implement `@import` in stylo.
+    }
+}