MDL-82160 renderer: Add render_context_header method to core_renderer

In MDL-81597 the original render_context_header method was removed and converted to a template.
That caused a regression, so this patch adds the method again but using the new template.

lib/outputrenderers.php

@@ -4644,6 +4644,17 @@ EOD;
         return $this->render($contextheader);
     }
 
+    /**
+     * Renders the header bar.
+     *
+     * @param context_header $contextheader Header bar object.
+     * @return string HTML for the header bar.
+     */
+    protected function render_context_header(context_header $contextheader) {
+        $context = $contextheader->export_for_template($this);
+        return $this->render_from_template('core/context_header', $context);
+    }
+
     /**
      * Renders the skip links for the page.
      *