Mark 5 virtual/threaded/transitions tests as flaky on debug bots

This patch marks the following tests as flaky on debug bots:
virtual/threaded/transitions/transition-end-event-multiple-01.html
virtual/threaded/transitions/transition-end-event-multiple-02.html
virtual/threaded/transitions/transition-end-event-unprefixed-04.html
virtual/threaded/transitions/transition-end-event-attributes.html
virtual/threaded/transitions/transition-end-event-window.html

These tests have been flaky for some time according to the
flakiness dashboard.

BUG=248938
TBR=eustas
NOTRY=true

Review URL: https://codereview.chromium.org/143653005

git-svn-id: svn://svn.chromium.org/blink/trunk@165507 bbb929c8-8fbe-4397-9dbb-9b2b20218538