Following [39272], this uses the `header_video_settings` filter to modify
the default video header control markup and adds theme specific styles
for the play/pause button.
Props melchoyce, laurelfulford, joemcgill, davidakennedy, bradyvercher.
Fixes#38697.
Built from https://develop.svn.wordpress.org/trunk@39273
git-svn-id: http://core.svn.wordpress.org/trunk@39213 1a063a9b-81f0-0310-95a4-ce76da25c4cd