Mark ServiceWorker postmessage-msgport-to-client.html as flaky

BUG=372260
TBR=kinuko@chromium.org

Review URL: https://codereview.chromium.org/277283002

git-svn-id: svn://svn.chromium.org/blink/trunk@173855 bbb929c8-8fbe-4397-9dbb-9b2b20218538