script: add skeleton implementation of FontFace API (#35262)

This patch implements the `FontFace` interface, but with some caveats

1. The interface is only exposed on `Window`. Support for Workers will
   be handled in the future.
2. The concept of `css-connected` `FontFace` is not implemented, so
   `@font-face` rules in stylesheets will not be represented in the DOM.
3. The constructor only supports using `url()` strings as source
   and `ArrayBuffer` and `ArrayBufferView` are not supported yet.

A skeleton implementation of the `load` method of `FontFaceSet` is also
implemented in this patch. The intention is to support some web pages
that don't load without this method.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/fonts/system_font_service.rs

@@ -565,7 +565,7 @@ impl SystemFontServiceProxy {
     }
 }
 
-#[derive(Clone, Debug, Default, Deserialize, Eq, Hash, PartialEq, Serialize)]
+#[derive(Clone, Debug, Default, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
 pub struct LowercaseFontFamilyName {
     inner: String,
 }