Remove has*Scrollbar and *Scrollbar from RenderLayer

These methods were seldom used outside RenderLayerScrollableArea
so made the callers aware of the RenderLayerScrollableArea.

BUG=260899

Review URL: https://codereview.chromium.org/26892002

git-svn-id: svn://svn.chromium.org/blink/trunk@159395 bbb929c8-8fbe-4397-9dbb-9b2b20218538