Restore output buffering of the page to support the legacy page cache system.

core/includes/bootstrap.inc

@@ -2326,6 +2326,10 @@ function _drupal_bootstrap_variables() {
  */
 function _drupal_bootstrap_page_header() {
   bootstrap_invoke_all('boot');
+
+  if (!drupal_is_cli()) {
+    ob_start();
+  }
 }
 
 /**