[resize-observer] "calculate box size" should specify it's using a ResizeObserverSize

[Loirooriol]

https://drafts.csswg.org/resize-observer-1/#calculate-box-size just starts like

Set computedSize.inlineSize to target’s [...]
[...]
return computedSize

But it never says what this computedSize is. I guess it's a ResizeObserverSize?

So the algorithm should start like

Let computedSize be a new ResizeObserverSize object.