DevTools: [CSS] do not suggest box-sizing: padding-box value

The "padding-box" value for box-sizing property is not currently supported
in chrome, so it should not be suggested in styles sidebar pane.

BUG=419900
R=apavlov, vsevik

Review URL: https://codereview.chromium.org/620323002

git-svn-id: svn://svn.chromium.org/blink/trunk@183195 bbb929c8-8fbe-4397-9dbb-9b2b20218538