gardening: mark http/tests/websocket/workers/close-code-and-reason.html as slow

BUG=391068
TBR=pilgrim@chromium.org, tyoshino@chromium.org

Review URL: https://codereview.chromium.org/364923003

git-svn-id: svn://svn.chromium.org/blink/trunk@177423 bbb929c8-8fbe-4397-9dbb-9b2b20218538