Gardening: Mark one test as flaky.

virtual/threaded/animations/animation-events-prefixed-03.html is flaky in Debug.

BUG=284531
TBR=tkent@chromium.org

Review URL: https://codereview.chromium.org/23530015

git-svn-id: svn://svn.chromium.org/blink/trunk@157142 bbb929c8-8fbe-4397-9dbb-9b2b20218538