Customizer: Improve IE 8 compatibility.

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

props westonruter, ocean90.
fixes #30781 for trunk.

git-svn-id: https://develop.svn.wordpress.org/trunk@30991 602fd350-edb4-49c9-b593-d223f7449a82