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.



git-svn-id: https://develop.svn.wordpress.org/trunk@27974 602fd350-edb4-49c9-b593-d223f7449a82