Update worker-storagequota-query-usage.html expectations

fast/workers/worker-storagequota-query-usage.html is flaky even on
debug, mark accordingly.

TBR=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/68993003

git-svn-id: svn://svn.chromium.org/blink/trunk@161731 bbb929c8-8fbe-4397-9dbb-9b2b20218538