Mark http/tests/xmlhttprequest/sync-repeated-and-caching.html as Flaky

BUG=327877
TBR=junov@chromium.org

Review URL: https://codereview.chromium.org/99903007

git-svn-id: svn://svn.chromium.org/blink/trunk@163740 bbb929c8-8fbe-4397-9dbb-9b2b20218538