[DevTools] Show extension lines without rulers in responsive design.

BUG=327641
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/341233002

git-svn-id: svn://svn.chromium.org/blink/trunk@176611 bbb929c8-8fbe-4397-9dbb-9b2b20218538