Flaky LayoutTest: virtual/threaded/animations/animation-iteration-event-destroy-renderer.html

BUG=248938
TBR=shawnsingh@chromium.org

Review URL: https://codereview.chromium.org/23664004

git-svn-id: svn://svn.chromium.org/blink/trunk@156879 bbb929c8-8fbe-4397-9dbb-9b2b20218538