Customizer: Use the `_.now()` function from Undescore.js for the current timestamp. `Date.now()` isn't supported in IE8.

fixes #27477.
Built from https://develop.svn.wordpress.org/trunk@27651


git-svn-id: http://core.svn.wordpress.org/trunk@27494 1a063a9b-81f0-0310-95a4-ce76da25c4cd