DevTools: [Styles] Do not select the first suggested CSS value item for empty input by default

The first suggested property value should not be auto-selected if the user input is empty (they might want to hit Tab to delete the property altogether).

R=pfeldman, vsevik
BUG=311584

Review URL: https://codereview.chromium.org/50703003

git-svn-id: svn://svn.chromium.org/blink/trunk@161447 bbb929c8-8fbe-4397-9dbb-9b2b20218538