DevTools: add a range control for better responsive design support

BUG=314367
R=kaznacheev@chromium.org

Review URL: https://codereview.chromium.org/45363005

git-svn-id: svn://svn.chromium.org/blink/trunk@161229 bbb929c8-8fbe-4397-9dbb-9b2b20218538