Mark frame-loading-via-document-write.html as still flaky

Though it now sometimes passes thanks to the rebaseline in r163524,
it often fails, so re-marking as flaky.

BUG=289002
TBR=mkwst@chromium.org

Review URL: https://codereview.chromium.org/111423004

git-svn-id: svn://svn.chromium.org/blink/trunk@163567 bbb929c8-8fbe-4397-9dbb-9b2b20218538