When dynamically instantiating `MediaElementPlayer` instances in JS, `preload="metadata"` is extremely wonky. `preload="none"` lets the file play right away. 

This has already been addressed for audio|video shortcodes, but also needs to be the case for playlists. When loading a playlist from a DOM cache or via AJAX, and when the sources match those already pre-loaded, the browser will sometimes donkey-punch the HTTP request for the first cached track selected.

Set `preload="none"` for the empty `<audio|video>` tag for playlists.

See commits related to #26779.


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git-svn-id: http://core.svn.wordpress.org/trunk@27804 1a063a9b-81f0-0310-95a4-ce76da25c4cd