Mark fast/js/JSON-stringify.html as flaky.

Started after V8 roll to version 3.22.23.

TBR=yurys@chromium.org
BUG=306929

Review URL: https://codereview.chromium.org/53573002

git-svn-id: svn://svn.chromium.org/blink/trunk@160976 bbb929c8-8fbe-4397-9dbb-9b2b20218538