fix: MediaSessionActionHandler cb's type

[marcoscaceres]

Fixes #322

Cc @saschanaz - how's this look?

Also, please see #331

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[saschanaz]

I don't think we can have multiple dictionaries in a union. Consider this:

({ action: "play", seekOffset: 3, seekTime: 4, fastSeek: true, isActivating: true })

This would fit any of the dictionaries in the union. For this specific case we don't do ES-to-IDL conversion so maybe it's theoretically fine?

cc @domenic

[youennf]

@saschanaz, good point.
I guess this could work for IDL-to-ES conversion, but would probably need updating WebIDL processors, which UAs might not do.
I am tempted to revert the splitting of dictionaries in the spec. @steimelchrome, thoughts?

[steimelchrome]

The fact that we can't just say MediaSessionActionDetails and allow 'subclass' dictionaries to be given feels like a strange limitation, but given that I'd agree with @youennf that reverting the dictionary split change makes the most sense

[youennf]

OK, let's close this PR, I'll work on a PR to revert the dictionary splitting.