[CSSRegions] Regions auto-height and absolute positioning bug

When laying out an absolutely positioned region with bottom: 0 and height: auto,
we have to update the regions position at the end of auto-height algorithm,
so that the regions updated height is properly taken into account.

Original WebKit patch (https://bugs.webkit.org/show_bug.cgi?id=111092) by Anton Obzhirov.

BUG=301503

Review URL: https://codereview.chromium.org/25077003

git-svn-id: svn://svn.chromium.org/blink/trunk@158785 bbb929c8-8fbe-4397-9dbb-9b2b20218538