Gardening: mark http/tests/accessibility/slow-document-load.html as flaky on Linux

TBR=hayato@chromium.org,dmazzoni@chromium.org
BUG=363236

Review URL: https://codereview.chromium.org/237093005

git-svn-id: svn://svn.chromium.org/blink/trunk@171496 bbb929c8-8fbe-4397-9dbb-9b2b20218538