Register default action for when no media session is active

[lucas42]

Some platforms have a default behaviour for when a media key is pressed but there is no media session currently active.
For example, pressing play on macOS with no active media will open the Music (formerly iTunes) application.
This default can be switched to other native applications, but I've not yet found a way for a web-based media player to be set as default.

I'd like to see a method similar to Navigator.registerProtocolHandler(), which would allow a web-based media player to be offered as the default for the play MediaSessionAction.

[My understanding is that current references to "default handler" in the spec only apply when there is an active media session. This new default would instead apply when no media session is active.]