DevTools: Make box-sizing model for ::before and ::after pseudos uniform with others.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/177003018

git-svn-id: svn://svn.chromium.org/blink/trunk@167911 bbb929c8-8fbe-4397-9dbb-9b2b20218538