Customizer: When navigating around the site within the Customizer preview, update the document title.

props westonruter.
fixes #28542.
Built from https://develop.svn.wordpress.org/trunk@30306


git-svn-id: http://core.svn.wordpress.org/trunk@30305 1a063a9b-81f0-0310-95a4-ce76da25c4cd