DevTools: make responsive design a master switch for emulation.

BUG=327641
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/341543003

git-svn-id: svn://svn.chromium.org/blink/trunk@176340 bbb929c8-8fbe-4397-9dbb-9b2b20218538