Cleanup after [29179]:

* Cleanup players when the editor is hidden - window scope is unique to each frame
* Add the editor body class to each iframe sandbox
* Remove unneeded code from `wp-mediaelement.js`

See #28905.

Built from https://develop.svn.wordpress.org/trunk@29189


git-svn-id: http://core.svn.wordpress.org/trunk@28973 1a063a9b-81f0-0310-95a4-ce76da25c4cd