Under this scenario a video might not fall back to the origin on pre-render.
Possible fix for ampproject#27109

Reproduces by navigating (locally to)
http://localhost:8000/proxy/s/stories.nonblocking.io/waffles/?usqp=mq331AQFKAGwASA%3D&amp_js_v=0.1#referrer=https%3A%2F%2Fwww.google.com&visibilityState=prerender&cap=swipe&ampshare=https%3A%2F%2Funumbox.com%2Fwebsite-development-trends-2020-amp-story%2F
waiting for the pre-render load to fail, and then executing `AMP.viewer.receiveMessage('visibilitychange', {'state': 'visible'})` in the console.

If this if not a fix for ampproject#27109, then it might be the same race, but elsewhere. The problem is that you cannot wait for `loadPromise` on a video element that has failed a source and has just had a new source provided.

- Change preload to also preload non-cache sources
- Clarify preconnect docs about the security invariants.