Reduce the usage of layer() inside RenderLayerScrollableArea

Ideally, RenderLayerScrollableArea should not depend on
RenderLayer so that they can be completely decoupled.

This change also removes 2 methods from RenderLayer
that were shims over the RenderLayerScrollableArea
implementation.

BUG=260899

Review URL: https://codereview.chromium.org/26582003

git-svn-id: svn://svn.chromium.org/blink/trunk@159278 bbb929c8-8fbe-4397-9dbb-9b2b20218538