Remove an extra relayout for non-percent logical height grid items
The only reason for a grid item to force a relayout after a grid
area's logical height change is if it has a percentage height. In
all other cases, this would cause an unneeded relayout. Note that
changing the logical width still forces a relayout as the situation
is not that simple.
This improves auto-grid-lots-of-data.html from 94 runs/s to 226
runs/s as we avoid a relayout in layoutGridItems for every grid