Call MediaElement's shim method for `setSrc()` when dynamically updating the playing source in playlists, which will properly set the source for mobile.

Make the labels for playlist items slightly smaller to account for size on mobile.

Fixes #27625.


Built from https://develop.svn.wordpress.org/trunk@27895


git-svn-id: http://core.svn.wordpress.org/trunk@27726 1a063a9b-81f0-0310-95a4-ce76da25c4cd