Sheriff: disable flaky ServiceWorkerOnStartupEventTest.Event test

Bug: 1243815
Change-Id: If70fda85752c3dec4ba4b2f7ad64eeb81ca09372
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3146670


Reviewed-by: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Commit-Queue: Jiewei Qian  <qjw@chromium.org>
Owners-Override: Jiewei Qian  <qjw@chromium.org>
Cr-Commit-Position: refs/heads/main@{#919128}