Move more of the composited scrolling code

While touching RenderLayerStackingNode,
made a private function private for real.

BUG=260899

Review URL: https://codereview.chromium.org/56613002

git-svn-id: svn://svn.chromium.org/blink/trunk@161271 bbb929c8-8fbe-4397-9dbb-9b2b20218538