Properly repaint grid item when z-index changes

Previously z-index changes of static positioned objects are ignored.
However, z-index is applicable for some static positioned objects,
such as grid items. Ignoring them makes the object not properly painted
on z-index change.

As StyleAdjuster has ensured that z-index is non-auto only if
applicable, RenderStyle::visualInvalidationDiff() should not check
again (with inconsistent conditions).

BUG=234200
TEST=fast/css-grid/layout/grid-item-z-index-change-repaint.html

Review URL: https://codereview.chromium.org/246113002

git-svn-id: svn://svn.chromium.org/blink/trunk@172168 bbb929c8-8fbe-4397-9dbb-9b2b20218538