[css-values-4] Should we ignore the scrollbars/etc when using vw/etc units on the root?

[tabatkins]

In #6026, we just resolved to subtract scrollbars from viewport units when overflow:scroll is specified on the root (or similar things, like scrollbar-gutter:always).

For obvious reasons, we do not do this adjustment when v* units are used in MQs; it's trivially cyclic. @emilio asked in IRC if we need to similarly ignore these adjustments for v* units used on the root, similarly to how we treat em and other "look at the parent for some information" units.

It wasn't immediately clear to us on the call if there's actually a potential cyclic issue here or not, so this issue is to figure out if there is.