DevTools: [Console] Do not bring up suggestbox automatically with no prefix

Restore the behavior modified in r154645.

R=vsevik@chromium.org
BUG=263925

Review URL: https://codereview.chromium.org/20709002

git-svn-id: svn://svn.chromium.org/blink/trunk@155086 bbb929c8-8fbe-4397-9dbb-9b2b20218538