DevTools: Extract String.naturalOrderComparator from ObjectPropertiesSection.

This comparator will be needed in other places too.

BUG=None
R=apavlov,yurys@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19519009

git-svn-id: svn://svn.chromium.org/blink/trunk@154496 bbb929c8-8fbe-4397-9dbb-9b2b20218538