Mark compositing/iframes/iframe-content-flipping as flaky in Debug

BUG=263120
R=cbiesinger@chromium.org

Review URL: https://codereview.chromium.org/19663012

git-svn-id: svn://svn.chromium.org/blink/trunk@154690 bbb929c8-8fbe-4397-9dbb-9b2b20218538