Revert 159283 "Move RenderLayerStackingNode out of RenderLayer."

> 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

TBR=dsinclair@chromium.org

Review URL: https://codereview.chromium.org/27057002

git-svn-id: svn://svn.chromium.org/blink/trunk@159465 bbb929c8-8fbe-4397-9dbb-9b2b20218538