Fix for broken overflow: overlay scrollbars introduced in 
https://chromiumcodereview.appspot.com/16982005

BUG=268232

Review URL: https://chromiumcodereview.appspot.com/22351005

git-svn-id: svn://svn.chromium.org/blink/trunk@155632 bbb929c8-8fbe-4397-9dbb-9b2b20218538