[CSS Regions] Use hasOverflowClip() in RenderRegion

Patch already landed in WebKit. See http://trac.webkit.org/changeset/159682

Implement the suggested FIXME in RenderRegion using hasOverflowClip().

Review URL: https://codereview.chromium.org/83273002

git-svn-id: svn://svn.chromium.org/blink/trunk@162591 bbb929c8-8fbe-4397-9dbb-9b2b20218538