Gardening: Mark http/tests/serviceworker/fetch-frame-resource.html as flaky

BUG=420307
TBR=horo@chromium.org,hayato@chromium.org

Review URL: https://codereview.chromium.org/630563002

git-svn-id: svn://svn.chromium.org/blink/trunk@183221 bbb929c8-8fbe-4397-9dbb-9b2b20218538