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.

Built from https://develop.svn.wordpress.org/branches/3.9@28273


git-svn-id: http://core.svn.wordpress.org/branches/3.9@28101 1a063a9b-81f0-0310-95a4-ce76da25c4cd