DevTools: [Styles] Do not bring property value prompt automatically after a stop character

A small refactoring has been made: autocompletion of empty input is now
delegated to the completion callbacks rather than decided upon by TextPrompt itself.

R=vsevik
BUG=264816

Review URL: https://chromiumcodereview.appspot.com/21232002

git-svn-id: svn://svn.chromium.org/blink/trunk@155165 bbb929c8-8fbe-4397-9dbb-9b2b20218538