The active media session might not be the only one being notified of actions

[youennf]

As discussed in #274 (comment), non active MediaSessions may receive notification of actions.
This for instance happens in Chrome when the user is interacting with browser UI that is unambiguously tied to a MediaSession (potentially not the active one).

The MediaSession spec is currently not aligned to Chrome implementation and it might be good to update the spec accordingly.
For instance https://w3c.github.io/mediasession/#handle-media-session-action will only trigger actions on the active media session and will abort at step 1 if the active media session is null.

One possibility is for the Media Session Action source to be tied to a specific Media Session.