Move RenderLayerStackingNode out of RenderLayer.

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

BUG=302120

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=159283

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

git-svn-id: svn://svn.chromium.org/blink/trunk@160012 bbb929c8-8fbe-4397-9dbb-9b2b20218538