Gardening: http/tests/media/progress-events-generated-correctly.html is flaky.

BUG=280342
TBR=wolenetz@chromium.org

Review URL: https://codereview.chromium.org/23620010

git-svn-id: svn://svn.chromium.org/blink/trunk@156918 bbb929c8-8fbe-4397-9dbb-9b2b20218538