LibWeb: Implement the CrossOriginPropertyFallback AO

Author: https://github.com/linusg
Commit: 34fed7bfcc
Pull-request: https://github.com/SerenityOS/serenity/pull/12919
Reviewed-by: https://github.com/awesomekling ✅

Userland/Libraries/LibWeb/Bindings/CrossOriginAbstractOperations.h

@@ -28,6 +28,7 @@ struct CrossOriginKey {
 using CrossOriginPropertyDescriptorMap = HashMap<CrossOriginKey, JS::PropertyDescriptor>;
 
 Vector<CrossOriginProperty> cross_origin_properties(Variant<LocationObject const*, WindowObject const*> const&);
+JS::ThrowCompletionOr<JS::PropertyDescriptor> cross_origin_property_fallback(JS::GlobalObject&, JS::PropertyKey const&);
 
 }