DevTools: [Styles] Display color swatch for editable styles

This got regressed in r164862.

R=aandrey, eustas, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/138173004

git-svn-id: svn://svn.chromium.org/blink/trunk@165067 bbb929c8-8fbe-4397-9dbb-9b2b20218538