Remove friendship from RenderLayerScrollableArea

It's time to strenghen the API between RenderLayer
and RenderLayerScrollableArea. This is the easy
friendship to break.

BUG=260899

Review URL: https://codereview.chromium.org/54603006

git-svn-id: svn://svn.chromium.org/blink/trunk@161129 bbb929c8-8fbe-4397-9dbb-9b2b20218538