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

R=pfeldman, vsevik
BUG=227438
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/18992002

git-svn-id: svn://svn.chromium.org/blink/trunk@154120 bbb929c8-8fbe-4397-9dbb-9b2b20218538