Customizer: Export Previewer instance to wp.customize.previewer, and utilize for Widget Customizer.

props westonruter.
fixes #27666.
Built from https://develop.svn.wordpress.org/trunk@29048


git-svn-id: http://core.svn.wordpress.org/trunk@28836 1a063a9b-81f0-0310-95a4-ce76da25c4cd