Gardening: Mark a test as flaky

virtual/threaded/transitions/transition-end-event-multiple-01.html
is flaky on Win release as well as debug.

BUG=248938
TBR=gavinp@chromium.org

Review URL: https://codereview.chromium.org/139853026

git-svn-id: svn://svn.chromium.org/blink/trunk@165789 bbb929c8-8fbe-4397-9dbb-9b2b20218538