DevTools: [Elements] Pseudo-elements not displayed for empty elements.

R=pfeldman, vsevik
BUG=298212

Review URL: https://chromiumcodereview.appspot.com/24735003

git-svn-id: svn://svn.chromium.org/blink/trunk@158373 bbb929c8-8fbe-4397-9dbb-9b2b20218538