Rather than extending `wp.media.mixin`, only borrow `coerce` when necessary in `wp.media.audio|video|collection`. `wp.media.mixin` will make sense to be mixed in for classes that expect to interact with the player. More universal methods can be added and inherited by all those who extend their prototype with it.

Assuming someone had implemented players in the editor, pause all players when switching between editor tabs. A method, `pauseAllPlayers`, has been added to `wp.media.mixin`.

See #27389.



git-svn-id: https://develop.svn.wordpress.org/trunk@27537 602fd350-edb4-49c9-b593-d223f7449a82