Remove layerForScrollChild / layerForScrolling

These methods were either never used or used
as part of the ScrollableArea interface.

BUG=260899

Review URL: https://codereview.chromium.org/26237008

git-svn-id: svn://svn.chromium.org/blink/trunk@159582 bbb929c8-8fbe-4397-9dbb-9b2b20218538