Move zIndex into RenderLayerStackingNode.

Nothing in RenderLayer uses the zIndex method, so move it to the stacking node.

BUG=302120

Review URL: https://codereview.chromium.org/30603002

git-svn-id: svn://svn.chromium.org/blink/trunk@160128 bbb929c8-8fbe-4397-9dbb-9b2b20218538