Videos in the media modal:

* Support rendering of chromeless YouTube when a video shortcode's `src` is a YouTube URL. 
* Don't instantiate an instance of `MediaElementPlayer` until after the view has been attached to the DOM and the view's `ready` event has fired.
* Don't set `poster` for videos when its value is empty. Much like `src` in the `img` tag - when empty, it will assume and load the current window's URL.
* When removing a player instance, don't call the `pause` method if the playback is not native.

See #27016.



git-svn-id: https://develop.svn.wordpress.org/trunk@27452 602fd350-edb4-49c9-b593-d223f7449a82