virtual/serviceworker/http/tests/serviceworker/fetch-event.html is flaky

BUG=381827
TBR=jsbell
NOTRY=true

Review URL: https://codereview.chromium.org/314413002

git-svn-id: svn://svn.chromium.org/blink/trunk@175695 bbb929c8-8fbe-4397-9dbb-9b2b20218538