Gardening: Widen flaky expectations for timeline-event-dispatch.html

BUG=407915
TBR=eustas@chromium.org

Review URL: https://codereview.chromium.org/627483002

git-svn-id: svn://svn.chromium.org/blink/trunk@183155 bbb929c8-8fbe-4397-9dbb-9b2b20218538