[CSS Regions] Mouse over an element does not trigger :hover state for parent when the element is flowed in a region

This re-implements https://chromiumcodereview.appspot.com/15773005 but fixes the problem that the initial patch generated: http://code.google.com/p/chromium/issues/detail?id=253472

NOTRY=true
(unrelated failing tests with and without patch)

BUG=262182

Review URL: https://chromiumcodereview.appspot.com/19492010

git-svn-id: svn://svn.chromium.org/blink/trunk@154597 bbb929c8-8fbe-4397-9dbb-9b2b20218538