DevTools: [Styles] Bring up suggest box for empty property values automatically

R=vsevik@chromium.org
BUG=227438

Review URL: https://codereview.chromium.org/18992002

git-svn-id: svn://svn.chromium.org/blink/trunk@154645 bbb929c8-8fbe-4397-9dbb-9b2b20218538