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

wp-includes/class-wp-customize-manager.php

@@ -575,6 +575,7 @@ final class WP_Customize_Manager {
 			// to operate properly.
 			$this->stop_previewing_theme();
 			switch_theme( $this->get_stylesheet() );
+			update_option( 'theme_switched_via_customizer', true );
 			$this->start_previewing_theme();
 		}