LibWeb: Make SessionHistoryEntry GC-allocated

These will need to float around more than they're currently able to.
Put them on the GC heap to prepare for that.

Co-authored-by: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>

Userland/Libraries/LibWeb/HTML/SessionHistoryEntry.cpp

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/HTML/SessionHistoryEntry.h>
+
+namespace Web::HTML {
+
+void SessionHistoryEntry::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(document);
+    visitor.visit(original_source_browsing_context);
+}
+
+}