CubebDeviceEnumerator already knows when an audio device changes. It is enhanced to allow listeners/observers registration and to create notifications when that happens. Also, it is hooked to the existing notification path.
On a minor note, it has been revisited the way the enumerator is touched in MediaEngineWebRTC class.
Differential Revision: https://phabricator.services.mozilla.com/D46271
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extra : moz-landing-system : lando