Instead of skipping the absolute and fixed child lists, we walk all kids of the
frame. But before recursing down into things that are absolute containing
blocks we ensure that we're only looking for fixed-pos placeholders, so we don't
reframe if we have a relatively positioned descendant with absolutely positioned
kids, for example. Note that this part is pure optimization attempt, and it
might be cheaper to not do it: IsAbsoluteContainingBlock is not that cheap and
the situations where we avoid reframing due to this optimization are likely
fairly rare.
