Fix persistence of <track> elements in the body of a [video] shotcode in TinyMCE views.

Merges [28183] and [28169] to the 3.9 branch.

props azaozz, wonderboymusic.
fixes #27915.


git-svn-id: https://develop.svn.wordpress.org/branches/3.9@28273 602fd350-edb4-49c9-b593-d223f7449a82