Persisting `<track>` elements as the body of a `[video]` shortcode in MCE Views:

* When generating the view's HTML, ensure that the shortcode's `content` is added to the model
* Add a `PostProcess` event in the `wpview` plugin to properly return the shortcode when the editor mode is toggled, ensuring that elements in the body are not dropped.

Props azaozz, wonderboymusic.
See #27915.




git-svn-id: https://develop.svn.wordpress.org/trunk@28183 602fd350-edb4-49c9-b593-d223f7449a82