Embeds: Correctly remove security attribute from iframes in IE 10 and IE 11.

This was originally added in 4.4, but presumably broke with [35708], which prevented these browsers from actually reaching the relevant code section.
Let's make embeds work again in IE 10 and IE 11.

Fixes #38694.
Built from https://develop.svn.wordpress.org/trunk@39347


git-svn-id: https://core.svn.wordpress.org/trunk@39287 1a063a9b-81f0-0310-95a4-ce76da25c4cd