LibWeb: Make HTMLCollection faster when it only cares about children

Some of the live HTMLCollection only ever contain children of their root
node. When we know that's the case, we can avoid doing a full subtree
traversal of all descendants and only visit children.

This cuts the ECMA262 spec loading time by over 10 seconds. :^)

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.cpp

@@ -19,7 +19,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<HTMLOptionsCollection>> HTMLOptionsCollecti
 }
 
 HTMLOptionsCollection::HTMLOptionsCollection(DOM::ParentNode& root, Function<bool(DOM::Element const&)> filter)
-    : DOM::HTMLCollection(root, move(filter))
+    : DOM::HTMLCollection(root, Scope::Descendants, move(filter))
 {
 }