Shave some yaks in RenderLayerStackingNode

This CL contains a number of minor cleanups to RenderLayerStackingNode. The
most interesting change is merging ancestorStackingContextNode and
ancestorStackingNode, which do exactly the same thing.

Review URL: https://codereview.chromium.org/301233008

git-svn-id: svn://svn.chromium.org/blink/trunk@175236 bbb929c8-8fbe-4397-9dbb-9b2b20218538