Gardening: mark virtual/threaded/transitions/transition-end-event-shorthands.html as flaky

BUG=248938
TBR=esprehn

Review URL: https://codereview.chromium.org/297273004

git-svn-id: svn://svn.chromium.org/blink/trunk@174806 bbb929c8-8fbe-4397-9dbb-9b2b20218538