insecure-eventsource-in-main-frame.html is flaky.

BUG=390358
TBR=mkwst@chromium.org

Review URL: https://codereview.chromium.org/367513004

git-svn-id: svn://svn.chromium.org/blink/trunk@177261 bbb929c8-8fbe-4397-9dbb-9b2b20218538