Issue #1831076 by ACF, Albert Volkman: Convert omit_vary_cookie() to cmi.

core/includes/bootstrap.inc

@@ -1391,7 +1391,7 @@ function drupal_serve_page_from_cache(stdClass $cache) {
   // response to reply to a subsequent request for a given URL without
   // revalidation. If a Vary header has been set in hook_boot(), it is assumed
   // that the module knows how to cache the page.
-  if (!isset($hook_boot_headers['vary']) && !variable_get('omit_vary_cookie')) {
+  if (!isset($hook_boot_headers['vary']) && !config('system.performance')->get('cache.page.omit_vary_cookie')) {
     header('Vary: Cookie');
   }
 

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

@@ -1,6 +1,7 @@
 cache:
   page:
     enabled: '0'
+    omit_vary_cookie: ''
     max_age: '0'
 css:
   preprocess: '0'

core/modules/system/system.install

@@ -1872,6 +1872,7 @@ function system_update_8017() {
     'page_compression' => 'response.gzip',
     'preprocess_css' => 'css.preprocess',
     'preprocess_js' => 'js.preprocess',
+    'omit_vary_cookie' => 'omit_vary_cookie',
     'stale_file_threshold' => 'stale_file_threshold',
   ));
 }

sites/default/default.settings.php

@@ -444,7 +444,7 @@ ini_set('session.cookie_lifetime', 2000000);
  * HTTP proxy, and bypass the reverse proxy if one is used) in order to avoid
  * getting cached pages from the proxy.
  */
-# $conf['omit_vary_cookie'] = TRUE;
+# $conf['system.performance']['cache']['page']['omit_vary_cookie'] = TRUE;
 
 /**
  * CSS/JS aggregated file gzip compression: