DevTools: beautify the responsive design warning message.

BUG=327641
NOTRY=true
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/343563003

git-svn-id: svn://svn.chromium.org/blink/trunk@176412 bbb929c8-8fbe-4397-9dbb-9b2b20218538