LibWeb: Implement caching of reflected element array attributes

For attributes like Element.ariaControlsElements, which are a reflection
of FrozenArray<Element>, we must return the same JS::Array object every
time the attribute is invoked - until its contents have changed. This
patch implements caching of the reflected array in accordance with the
spec.

Libraries/LibWeb/WebIDL/AbstractOperations.cpp

@@ -528,4 +528,25 @@ template JS::ThrowCompletionOr<UnsignedLong> convert_to_int(JS::VM& vm, JS::Valu
 template JS::ThrowCompletionOr<LongLong> convert_to_int(JS::VM& vm, JS::Value, EnforceRange, Clamp);
 template JS::ThrowCompletionOr<UnsignedLongLong> convert_to_int(JS::VM& vm, JS::Value, EnforceRange, Clamp);
 
+// AD-HOC: For same-object caching purposes, this can be used to compare a cached JS array of DOM::Elements with another
+//         list. Either list can be null, in which case they are considered the same only if they are both null.
+bool lists_contain_same_elements(GC::Ptr<JS::Array> array, Optional<GC::RootVector<GC::Ref<DOM::Element>>> const& elements)
+{
+    if (!array || !elements.has_value())
+        return !array && !elements.has_value();
+
+    bool is_equivalent = array->indexed_properties().array_like_size() == elements->size();
+
+    for (size_t i = 0; is_equivalent && i < elements->size(); ++i) {
+        auto cached_value = array->get_without_side_effects(JS::PropertyKey { i });
+        auto const& cached_element = as<DOM::Element>(cached_value.as_object());
+
+        auto it = elements->find_if([&](auto const& element) { return element.ptr() == &cached_element; });
+        if (it == elements->end())
+            is_equivalent = false;
+    }
+
+    return is_equivalent;
+}
+
 }