Only RenderBoxes should have a RenderLayerScrollableArea

This change updates the RenderLayer code to only allocate
RenderLayerScrollableArea for RenderBoxes. This enables us
to simplify RenderLayerScrollableArea based on the knowledge
that the associated renderer is a RenderBox.

BUG=260899

Review URL: https://chromiumcodereview.appspot.com/24653004

git-svn-id: svn://svn.chromium.org/blink/trunk@158458 bbb929c8-8fbe-4397-9dbb-9b2b20218538