DevTools: Add support for the pseudo :before and :after elements.

R=pfeldman, vsevik
BUG=180291

Review URL: https://chromiumcodereview.appspot.com/21898003

git-svn-id: svn://svn.chromium.org/blink/trunk@158030 bbb929c8-8fbe-4397-9dbb-9b2b20218538