Mark fast/frames/sandboxed-iframe-navigation-windowopen.html as flaky on windows

It times out from time to time

BUG=321793
TEST=
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/79293003

git-svn-id: svn://svn.chromium.org/blink/trunk@162414 bbb929c8-8fbe-4397-9dbb-9b2b20218538