Revert treat mixed XHR as active content.

BUG=305303
R=abarth@chromum.org,cevans@chromium.org

Review URL: https://codereview.chromium.org/26539002

git-svn-id: svn://svn.chromium.org/blink/trunk@159186 bbb929c8-8fbe-4397-9dbb-9b2b20218538