[cssom-view] potentially scrollable should check computed value, not used value

[zcorpan]

https://drafts.csswg.org/cssom-view/#terminology

The element is not the HTML body element, or it is and the root element’s used value of the overflow-x or overflow-y properties is not visible.
The element’s used value of the overflow-x or overflow-y properties is not visible.

Combining this with:

UAs must apply the overflow property set on the root element to the viewport. For HTML UAs, if this would result in the viewport having visible overflow, the UA must instead apply the overflow property set on the body element to the viewport. In either case, the used value of overflow for the element from which the value is propagated must evaluate to visible.

https://drafts.csswg.org/css-overflow-3/#overflow-propagation

means at least one of html or body will have used value 'visible'.

The spec should check the computed value instead.

cc @rune-opera