Mark various compositing tests as flaky on Mac

Unreviewed gardening.

BUG=310679
TBR=pfeldman@chromium.org,ccameron@chromium.org

Review URL: https://codereview.chromium.org/37503003

git-svn-id: svn://svn.chromium.org/blink/trunk@160350 bbb929c8-8fbe-4397-9dbb-9b2b20218538