Don't always repaint on zIndex change

If zIndex of a composited layer changes and there is no other conditions
triggering repaint, we should not repaint the layer.

Now treat zIndex change like opacity change about repainting.

BUG=364636
TEST=reuse existing compositing/repaint/should-not-repaint-composited-descendants.html

Review URL: https://codereview.chromium.org/240253007

git-svn-id: svn://svn.chromium.org/blink/trunk@171989 bbb929c8-8fbe-4397-9dbb-9b2b20218538