Issue #1824762 by justafish, alexpott: Convert admin_compact_mode() to CMI.

core/modules/system/config/system.site.yml

@@ -5,3 +5,4 @@ page:
   403: ''
   404: ''
   front: user
+admin_compact_mode: '0'

core/modules/system/system.install

@@ -2220,6 +2220,17 @@ function system_update_8035() {
   ));
 }
 
+/**
+ * Move the admin_compact_mode setting from variable to config.
+ *
+ * @ingroup config_upgrade
+ */
+function system_update_8036() {
+  update_variables_to_config('system.site', array(
+    'admin_compact_mode' => 'admin_compact_mode',
+  ));
+}
+
 /**
  * @} End of "defgroup updates-7.x-to-8.x".
  * The next series of updates should start at 9000.

core/modules/system/system.module

@@ -3379,7 +3379,7 @@ function confirm_form($form, $question, $path, $description = NULL, $yes = NULL,
 function system_admin_compact_mode() {
   // PHP converts dots into underscores in cookie names to avoid problems with
   // its parser, so we use a converted cookie name.
-  return isset($_COOKIE['Drupal_visitor_admin_compact_mode']) ? $_COOKIE['Drupal_visitor_admin_compact_mode'] : variable_get('admin_compact_mode', FALSE);
+  return isset($_COOKIE['Drupal_visitor_admin_compact_mode']) ? $_COOKIE['Drupal_visitor_admin_compact_mode'] : config('system.site')->get('admin_compact_mode');
 }
 
 /**