layout: Address review feedback.
test: Fix some reftests.
layout: Fix percentages in relatively positioned elements
layout: Address some more review comments
layout: Address review comments for `min-height` and `max-height`
computation
Adress more #1988 (Acid 2) review comments.
Computed value of 'height: <percentage>' is 'auto' in some cases.

This is necessary but not sufficient for #2029.

A bug fix in the previous commit "broke" Acid 2 because that bug was hiding
this one. This makes Acid 2 pass again.