In the Customizer a notice will show up if there is at least one registered sidebar with its section not being active. In order to achieve that, the count of registered sidebars is compared to the count of active sections in the Widgets panel. Prior to this change, the latter would incorrectly include any sections, even additional sections that do not correspond to a sidebar.
Fixes#43556.
Built from https://develop.svn.wordpress.org/trunk@43588
git-svn-id: http://core.svn.wordpress.org/trunk@43417 1a063a9b-81f0-0310-95a4-ce76da25c4cd
