Customize: Fix image cropping when doing live preview of theme switches.

Ensure that the Customizer gets bootstrapped with the pre-activated `theme` supplied in the `custom-header-crop` admin ajax requests.

Props faishal, rittesh.patel.
Fixes #32783.

Built from https://develop.svn.wordpress.org/trunk@36866


git-svn-id: http://core.svn.wordpress.org/trunk@36833 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-controls.js

@@ -2346,6 +2346,10 @@
 				api.HeaderTool.UploadsList,
 				api.HeaderTool.DefaultsList
 			]);
+
+			// Ensure custom-header-crop Ajax requests bootstrap the Customizer to activate the previewed theme.
+			wp.media.controller.Cropper.prototype.defaults.doCropArgs.wp_customize = 'on';
+			wp.media.controller.Cropper.prototype.defaults.doCropArgs.theme = api.settings.theme.stylesheet;
 		},
 
 		/**