Move RenderLayerStackingNode out of RenderLayer.

This CL extracts the code related to stacking out of render layer and
into a separate class.

BUG=302120

Review URL: https://codereview.chromium.org/25968003

git-svn-id: svn://svn.chromium.org/blink/trunk@159283 bbb929c8-8fbe-4397-9dbb-9b2b20218538