[geometry] DOMRectReadOnly serializer

https://drafts.fxtf.org/geometry/#DOMRect <https://drafts.fxtf.org/geometry/#DOMRect>

I think it’s a bit weird that DOMRectReadOnly uses "serializer = { attribute }”, since that will serialize all of { x, y, width, height, top, right, bottom, left }. I think it would make more sense to only serialize { x, y, width, height } since those are the underlying data.

Simon