Let the video shortcode accept a YouTube URL as the value of its `src`...

Let the video shortcode accept a YouTube URL as the value of its `src` attribute, as MediaElement.js supports Chromeless YouTube videos by using a pseudo-mime-type `video/youtube`. 

HTTP and HTTPS www, non-www, and short url fronts are supported:
http://www.youtube.com/watch 
https://www.youtube.com/watch
http://youtube.com/watch
https://youtube.com/watch
http://youtu.be
https://youtu.be

Fixes #24764.


Built from https://develop.svn.wordpress.org/trunk@27063


git-svn-id: https://core.svn.wordpress.org/trunk@26936 1a063a9b-81f0-0310-95a4-ce76da25c4cd