Add a privacy/security section

[foolip]

A MediaSession interacts with global state on the system, and if you have an active session that is interrupted, you make some guesses about what's going on. A short transient interruption is likely some kind of notification, a long transient interruption is probably a phone call.

We should also review whether MediaSession should be restricted to secure contexts.

(Inspired by @cbentzel on blink-dev, we can't ship without thinking this through.)

[xxyzzzq]

Closing this issue since we are moving audio focus out to a separate API.

[xxyzzzq]

This issue was moved to WICG/audio-focus#14