Height definiteness is now preserved as intended by CSS-SIZING-3
(assuming I've understood it correctly) and not implicitly granted by
layout algorithms when they assign height.
For the specific special/magical cases where some sizes become definite
during layout, the preceding commits have made them explicit in code.
This fixes a number of flex layout issues where we were previously
resolving percentage values against post-layout flex container heights,
but other browsers don't.