Widget Customizer: Fix widget filtering.

props westonruter.
fixes #27451.
Built from https://develop.svn.wordpress.org/trunk@28044


git-svn-id: http://core.svn.wordpress.org/trunk@27874 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Precommit cleanup for theme.js, editor.css amd wpdialogs/plugin.js, See #27055, #26907, #16284
Built from https://develop.svn.wordpress.org/trunk@28045


git-svn-id: http://core.svn.wordpress.org/trunk@27875 1a063a9b-81f0-0310-95a4-ce76da25c4cd