Customizer: Improve IE 8 compatibility.

* Use square bracket notation for `default` keywords.
* Use Underscores `indexOf` for arrays.

props westonruter, ocean90.
fixes #30781 for trunk.
Built from https://develop.svn.wordpress.org/trunk@30991


git-svn-id: http://core.svn.wordpress.org/trunk@30973 1a063a9b-81f0-0310-95a4-ce76da25c4cd