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

Merges [28182] to the 3.9 branch.

props gcorne, wonderboymusic.
fixes #27899.

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git-svn-id: http://core.svn.wordpress.org/branches/3.9@28100 1a063a9b-81f0-0310-95a4-ce76da25c4cd