#226728 by chx and Eaton: Flush form caches when cache is cleared on cron.

modules/system/system.module

@@ -1412,6 +1412,11 @@ function system_cron() {
     }
     db_query('DELETE FROM {files} WHERE fid = %d', $file->fid);
   }
+  $core = array('cache', 'cache_block', 'cache_filter', 'cache_page', 'cache_form', 'cache_menu');
+  $cache_tables = array_merge(module_invoke_all('flush_caches'), $core);
+  foreach ($cache_tables as $table) {
+    cache_clear_all(NULL, $table);
+  }
 }
 
 /**