LibWeb: Avoid full DOM tree traversal in supported_property_names()

Instead, take advantage of `ElementByIdMap` and
`m_potentially_named_elements` to gather named elements more
efficiently.

Author: kalenikaliaksandr

Libraries/LibWeb/DOM/ElementByIdMap.h

@@ -17,6 +17,13 @@ class ElementByIdMap {
     void remove(FlyString const& element_id, Element&);
     GC::Ptr<Element> get(FlyString const& element_id) const;
 
+    template<typename Callback>
+    void for_each_id(Callback callback)
+    {
+        for (auto const& id : m_map.keys())
+            callback(id);
+    }
+
 private:
     HashMap<FlyString, Vector<GC::Weak<Element>>> m_map;
 };

Libraries/LibWeb/HTML/Window.cpp

@@ -1826,14 +1826,12 @@ Vector<FlyString> Window::supported_property_names() const
     //   that have a non-empty name content attribute and are in a document tree with window's associated Document as their root; and
     // - the value of the id content attribute for all HTML elements that have a non-empty id content attribute
     //   and are in a document tree with window's associated Document as their root.
-    associated_document().for_each_in_subtree_of_type<DOM::Element>([&property_names](auto& element) -> TraversalDecision {
-        if (is<HTMLEmbedElement>(element) || is<HTMLFormElement>(element) || is<HTMLImageElement>(element) || is<HTMLObjectElement>(element)) {
-            if (element.name().has_value())
-                property_names.set(element.name().value(), AK::HashSetExistingEntryBehavior::Keep);
-        }
-        if (auto const& name = element.id(); name.has_value())
-            property_names.set(name.value().to_string(), AK::HashSetExistingEntryBehavior::Keep);
-        return TraversalDecision::Continue;
+    for (auto element : associated_document().potentially_named_elements()) {
+        if (auto name = element->name(); name.has_value())
+            property_names.set(*name, AK::HashSetExistingEntryBehavior::Keep);
+    }
+    associated_document().element_by_id().for_each_id([&](auto id) {
+        property_names.set(id, AK::HashSetExistingEntryBehavior::Keep);
     });
 
     return property_names.values();