Enable clip-path on composited overflow scrolling.

Currently clip-path is pretty broken on accelerated overflow divs,
causing invalidation issues, and preventing the background and
scrollbars from being clipped. This CL should fix all these
issues, except that in some cases scroll children may still
escape outside of the clip (see crbug.com/365828).

BUG=367165

Review URL: https://codereview.chromium.org/283953003

git-svn-id: svn://svn.chromium.org/blink/trunk@174048 bbb929c8-8fbe-4397-9dbb-9b2b20218538