No more underlay scrollbars

This CL ensures that overlay scrollbars stack above scrolling contents.

BUG=390332

Review URL: https://codereview.chromium.org/372513002

git-svn-id: svn://svn.chromium.org/blink/trunk@177684 bbb929c8-8fbe-4397-9dbb-9b2b20218538