SheriffOMatic: Add ct-popout-iframe and switch test output iframes to use it.

BUG=413067
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/559843006

git-svn-id: svn://svn.chromium.org/blink/trunk@181902 bbb929c8-8fbe-4397-9dbb-9b2b20218538