Mark fast/css/font-face-download-error.html as flaky

BUG=304953
R=enne@chromium.org

Review URL: https://codereview.chromium.org/26320004

git-svn-id: svn://svn.chromium.org/blink/trunk@159048 bbb929c8-8fbe-4397-9dbb-9b2b20218538