Correction to layoutOverflowRect calculation in case of direction:rtl

BUG=179332
Webkit bug https://bugs.webkit.org/show_bug.cgi?id=85856

Review URL: https://codereview.chromium.org/25373006

git-svn-id: svn://svn.chromium.org/blink/trunk@161333 bbb929c8-8fbe-4397-9dbb-9b2b20218538