Media Session doesn't work without <audio> element

[mmontag]

The Media Session sample is missing a small but important detail from explanation; the Media Session requires an active audio or video element. In the code and other documentation, this is not explicitly mentioned.

Hopefully this isn't always the case in the future. I filed a Chromium issue here: https://bugs.chromium.org/p/chromium/issues/detail?id=944538