Revert of Don't always repaint on zIndex change (https://codereview.chromium.org/240253007/)

Reason for revert:
Depends on https://codereview.chromium.org/203463007/ which broke ChromeOS login window: https://code.google.com/p/chromium/issues/detail?id=365507

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=171989

TBR=esprehn@chromium.org,eseidel@chromium.org,ojan@chromium.org,wangxianzhu@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=364636

Review URL: https://codereview.chromium.org/247293006

git-svn-id: svn://svn.chromium.org/blink/trunk@172172 bbb929c8-8fbe-4397-9dbb-9b2b20218538