Code review comment for lp:~unity-team/unity-api/music_cards_extra_attributes

Had a scan over this along with the associated changes in:

https://code.launchpad.net/~unity-team/unity8/audioCardSupport/+merge/269029
and https://code.launchpad.net/~stolowski/unity-scopes-shell/audio-card/+merge/267341

Looks good to me. Although, I am a an outsider regarding this development, so best to confirm this review with someone directly involved.