DevTools: Blackbox content scripts - backend support.

BUG=160207
R=vsevik, yurys@chromium.org

Review URL: https://codereview.chromium.org/547603003

git-svn-id: svn://svn.chromium.org/blink/trunk@181561 bbb929c8-8fbe-4397-9dbb-9b2b20218538