Intent to prototype: Audio Session API

[Alastor Wu]

Summary:

The Audio Session API lets web apps declare their audio intent and observe the system audio state.

Instead of relying on browser guesses, pages can specify whether they're playing media, a notification, or a call, and receive events when audio is interrupted, deactivated, or resumed. This makes web audio behave more like native apps across platforms. 

Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=2055710 (enable pref on Nightly)
https://bugzilla.mozilla.org/show_bug.cgi?id=1830428 (meta)

Specification: https://w3c.github.io/audio-session/

Preference: 

dom.audio_session.enabled

Platform coverage: All

Other browsers:  

https://caniuse.com/mdn-api_audiosession  
Chromium: Not supported 

Safari: Supported

Web-platform-tests:
https://wpt.fyi/results/audio-session