Issue #1824854 by cam8001: Convert variable drupal_stale_file_threshold() to CMI system.

core/includes/common.inc

@@ -3455,7 +3455,7 @@ function drupal_clear_css_cache() {
  */
 function drupal_delete_file_if_stale($uri) {
   // Default stale file threshold is 30 days.
-  if (REQUEST_TIME - filemtime($uri) > variable_get('drupal_stale_file_threshold', 2592000)) {
+  if (REQUEST_TIME - filemtime($uri) > config('system.performance')->get('preprocess.stale_file_threshold')) {
     file_unmanaged_delete($uri);
   }
 }

core/modules/system/config/system.performance.yml

@@ -5,5 +5,6 @@ cache:
 preprocess:
   css: '0'
   js: '0'
+  stale_file_threshold: '2592000'
 response:
   gzip: '0'

core/modules/system/system.install

@@ -1909,6 +1909,7 @@ function system_update_8017() {
     'page_compression' => 'response.gzip',
     'preprocess_css' => 'preprocess.css',
     'preprocess_js' => 'preprocess.js',
+    'stale_file_threshold' => 'preprocess.stale_file_threshold',
   ));
 }