Issues:
* This class is not a delegate (see https://en.wikipedia.org/wiki/Delegation_pattern)
* WMPI has a delegate_id which sounds like it's the id of the delegate, but it's WMPI's id. 
* RWMPD has a CleanupIdleDelegates. So the delegate considers the players to be delegates.
* The delegate methods take delegate_ids but the docs say things like "specified player". We should probably s/delegate_id/player_id/.
* The docs on the WMPD::Observer interface talk about WebMediaPlayers instead of Observers. Should probably s/Observer/Player.
* RendererWebMediaPlayerDelegateTest has a RWMPD member called delegate_manager_ but the type doesn't have 'manager' in the name.

I'm thinking we just call this MediaPlayerManager. + people who might care.