Test that we repaint contents in iframes

Turns out we don't have any test coverage for this case.

R=leviw@chromium.org
BUG=403734

Review URL: https://codereview.chromium.org/474373002

git-svn-id: svn://svn.chromium.org/blink/trunk@180394 bbb929c8-8fbe-4397-9dbb-9b2b20218538