Fix WebIDL mixin attributes on Window

Previously the "container attribute" were set to the attributes of the
mixin itself, but we want the container attributes to be that of the
type which includes the mixin (like `Window`) as those attributes
contain information about whether or not bindings are `structural`.

The end result with this is that the `structural` tag is now used for
properties on `Window`, correctly generating setters/getters.

Closes #904

crates/webidl/src/lib.rs

@@ -568,7 +568,7 @@ impl<'src> FirstPassRecord<'src> {
                     &member.type_,
                     member.identifier.0,
                     &member.attributes,
-                    mixin_data.definition_attributes,
+                    data.definition_attributes,
                 );
             }
         }