Move XHR helpers from media to utils. See #24424.


git-svn-id: https://core.svn.wordpress.org/trunk@24369 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Compress scripts/styles: 3.6-beta3-24369.

git-svn-id: https://core.svn.wordpress.org/trunk@24370 1a063a9b-81f0-0310-95a4-ce76da25c4cd