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.



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