Mark a test as flaky

This test is flaky on Linux, Linux debug and Linux ASAN.
virtual/threaded/transitions/transition-end-event-unprefixed-04.html

TBR=jchaffraix
BUG=248938

Review URL: https://codereview.chromium.org/131103015

git-svn-id: svn://svn.chromium.org/blink/trunk@165592 bbb929c8-8fbe-4397-9dbb-9b2b20218538