LibWeb: Return a representation of an 'Agent' in 'relevant agent'

This makes it more convenient to use the 'relvant agent' concept,
instead of the awkward dynamic casts we needed to do for every call
site.

mutation_observers is also changed to hold a GC::Root instead of raw
GC::Ptr. Somehow this was not causing problems before, but trips up CI
after these changes.

Libraries/LibWeb/HTML/CustomElements/CustomElementReactionsStack.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021-2023, Luke Wilde <lukew@serenityos.org>
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <LibGC/Root.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reactions-stack
+struct CustomElementReactionsStack {
+    CustomElementReactionsStack() = default;
+    ~CustomElementReactionsStack() = default;
+
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#element-queue
+    // Each item in the stack is an element queue, which is initially empty as well. Each item in an element queue is an element.
+    // (The elements are not necessarily custom yet, since this queue is used for upgrades as well.)
+    Vector<Vector<GC::Root<DOM::Element>>> element_queue_stack;
+
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#backup-element-queue
+    // Each custom element reactions stack has an associated backup element queue, which an initially-empty element queue.
+    Vector<GC::Root<DOM::Element>> backup_element_queue;
+
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#processing-the-backup-element-queue
+    // To prevent reentrancy when processing the backup element queue, each custom element reactions stack also has a processing the backup element queue flag, initially unset.
+    bool processing_the_backup_element_queue { false };
+};
+
+}