Mark virtual/threaded/transitions/transition-end-event-unprefixed-01.html as flaky.

Now on Linux.

TBR=caseq
BUG=248938

Review URL: https://codereview.chromium.org/150773002

git-svn-id: svn://svn.chromium.org/blink/trunk@166174 bbb929c8-8fbe-4397-9dbb-9b2b20218538