Gardening: Mark http/tests/xmlhttprequest/xmlhttprequest-allowed-with-disabled-web-security.html as flaky

BUG=420494
TBR=jsbell@chromium.org

Review URL: https://codereview.chromium.org/631673003

git-svn-id: svn://svn.chromium.org/blink/trunk@183246 bbb929c8-8fbe-4397-9dbb-9b2b20218538