Hold worker reference to prevent GC in flaky test

TBR=dgrogan@chromium.org
BUG=338866

Review URL: https://codereview.chromium.org/154953002

git-svn-id: svn://svn.chromium.org/blink/trunk@166533 bbb929c8-8fbe-4397-9dbb-9b2b20218538