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

Servo doesn't compile at this stage.

tests/unit/style/viewport.rs

@@ -25,6 +25,7 @@ macro_rules! stylesheet {
             ServoUrl::parse("http://localhost").unwrap(),
             Origin::$origin,
             Default::default(),
+            None,
             $error_reporter,
             ParserContextExtraData::default()
         ))