Move ancestorStacking methods into RenderLayerStackingNode.

This CL moves the ancestorStacking{Node|ContainerNode} and the
enclosingStackingContainerNode method into the RenderLayerStackingNode class.

BUG=302120

Review URL: https://codereview.chromium.org/29103008

git-svn-id: svn://svn.chromium.org/blink/trunk@160215 bbb929c8-8fbe-4397-9dbb-9b2b20218538