Ensure that we've allocated a positive z-order list before we add to it.

In RenderLayer::rebuildZOrderLists, we blindly append to posZOrder list, counting
on collectLayers having allocated it. But if there are no additions to that list
in collectLayers, it will still be null and we will crash. This patch allocates
the list if necessary.

BUG=306096
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/26742007

git-svn-id: svn://svn.chromium.org/blink/trunk@159491 bbb929c8-8fbe-4397-9dbb-9b2b20218538