LibWeb: Avoid full DOM tree traversal in Window::named_objects()

Instead, take advantage of `ElementByIdMap` and
`m_potentially_named_elements` to gather elements with specified name
more efficiently.

Author: kalenikaliaksandr

Libraries/LibWeb/DOM/ElementByIdMap.h

@@ -24,6 +24,18 @@ class ElementByIdMap {
             callback(id);
     }
 
+    template<typename Callback>
+    void for_each_element_with_id(StringView id, Callback callback)
+    {
+        auto maybe_elements_with_id = m_map.get(id);
+        if (!maybe_elements_with_id.has_value())
+            return;
+        for (auto const& element : *maybe_elements_with_id) {
+            if (element)
+                callback(GC::Ref { *element });
+        }
+    }
+
 private:
     HashMap<FlyString, Vector<GC::Weak<Element>>> m_map;
 };

Libraries/LibWeb/HTML/Window.cpp

@@ -1901,13 +1901,16 @@ Window::NamedObjects Window::named_objects(StringView name)
     // embed, form, img, or object elements that have a name content attribute whose value is name
     // and are in a document tree with window's associated Document as their root; and
     // HTML elements that have an id content attribute whose value is name and are in a document tree with window's associated Document as their root.
-    associated_document().for_each_in_subtree_of_type<DOM::Element>([&objects, &name](auto& element) -> TraversalDecision {
-        if ((is<HTMLEmbedElement>(element) || is<HTMLFormElement>(element) || is<HTMLImageElement>(element) || is<HTMLObjectElement>(element))
-            && (element.name() == name))
-            objects.elements.append(element);
-        else if (element.id() == name)
-            objects.elements.append(element);
-        return TraversalDecision::Continue;
+    for (auto element : associated_document().potentially_named_elements()) {
+        if (element->id().has_value()) {
+            // NOTE: The element will be added when we iterate over the element_by_id() map below.
+            continue;
+        }
+        if (auto element_name = element->name(); element_name.has_value() && *element_name == name)
+            objects.elements.append(*element);
+    }
+    associated_document().element_by_id().for_each_element_with_id(name, [&](auto element) {
+        objects.elements.append(*element);
     });
 
     return objects;