DevTools: [CSS] cleaning-up CSS Domain in protocol.json

This patch slightly cleans-up CSS Domain in protocol.json without
affecting inspector front-end.

BUG=

Review URL: https://codereview.chromium.org/142413003

git-svn-id: svn://svn.chromium.org/blink/trunk@166035 bbb929c8-8fbe-4397-9dbb-9b2b20218538