[CSS Grid Layout] Speed up content sized track layout
This change narrows down the relayout condition in
RenderGrid::logicalContentHeightForChild to only
percentage children: as they are the only lengths that
depends on the containing block's override. We
unfortunately can't use percentHeightDescendants as
it's not populated yet. This code can probably be
simplified further down but it's a good start.
This greatly speeds up the new performance test, moving
from 25 runs/sec to 91 runs/sec.