Customizer: Properly handle widget settings when activating a previewed theme.

props westonruter, ocean90, gcorne.
fixes #27767.

Built from https://develop.svn.wordpress.org/trunk@28124


git-svn-id: http://core.svn.wordpress.org/trunk@27955 1a063a9b-81f0-0310-95a4-ce76da25c4cd