Gardening: http/tests/webfont/slow-loading.html is flaky

BUG=371725
TBR=ksakamoto

Review URL: https://codereview.chromium.org/275813002

git-svn-id: svn://svn.chromium.org/blink/trunk@173740 bbb929c8-8fbe-4397-9dbb-9b2b20218538