Refinements for asynchronous rendering in `wp.mce.media.PlaylistView`.

Merges [28182] to the 3.9 branch.

props gcorne, wonderboymusic.
fixes #27899.


git-svn-id: https://develop.svn.wordpress.org/branches/3.9@28272 602fd350-edb4-49c9-b593-d223f7449a82