LibHTML: Don't bother updating layout in frameless documents

Before/after the document is attached to a frame, there's no point in
doing any kind of layout, since we can't display the results anywhere.

Libraries/LibHTML/DOM/Document.cpp

@@ -187,6 +187,9 @@ void Document::update_style()
 
 void Document::update_layout()
 {
+    if (!frame())
+        return;
+
     layout();
     if (on_layout_updated)
         on_layout_updated();