Fix rendering regression for out-of-process iframes

Changes to the compositing path meant the toggle of an iframe to having
a self-painting layer wasn't getting updated.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/330283008

git-svn-id: svn://svn.chromium.org/blink/trunk@176130 bbb929c8-8fbe-4397-9dbb-9b2b20218538