A region with non-visible overflow passes hasOverflowClip()

A renderer will only clip overflow and not pass hasOverflowClip()
if it is the primary <body> and propagates it to the viewport. This
will never be the case for a region.

BUG=

Review URL: https://codereview.chromium.org/132923007

git-svn-id: svn://svn.chromium.org/blink/trunk@166412 bbb929c8-8fbe-4397-9dbb-9b2b20218538