Widen expectations for flaky virtual/threaded/transitions/ tests

The following have been failing on Linux for a few weeks:
virtual/threaded/transitions/transition-end-event-set-none.html
virtual/threaded/transitions/transition-end-event-multiple-02.html

BUG=248938
TBR=caseq

Review URL: https://codereview.chromium.org/133533009

git-svn-id: svn://svn.chromium.org/blink/trunk@166286 bbb929c8-8fbe-4397-9dbb-9b2b20218538