Widget Customizer: Don't use opacity for the update spinner.

`opacity` isn't supported in IE8 and makes the spinner always visible.

see #27485.
Built from https://develop.svn.wordpress.org/trunk@27912


git-svn-id: http://core.svn.wordpress.org/trunk@27743 1a063a9b-81f0-0310-95a4-ce76da25c4cd