DevTools: Use "::" rather than ":" as the "before"/"after" pseudo element prefix

R=pfeldman, vsevik
BUG=302551

Review URL: https://codereview.chromium.org/25556002

git-svn-id: svn://svn.chromium.org/blink/trunk@158647 bbb929c8-8fbe-4397-9dbb-9b2b20218538