DevTools: [Elements] Implement the "Hide element" shortcut for ::before and ::after

R=aandrey, eustas, vsevik
BUG=323966

Review URL: https://codereview.chromium.org/95343002

git-svn-id: svn://svn.chromium.org/blink/trunk@162892 bbb929c8-8fbe-4397-9dbb-9b2b20218538