LibWeb: Allow HTML parser to delay delivery of the document "load" event

We will now spin in "the end" until there are no more "things delaying
the load event". Of course, nothing actually uses this yet, and there
are a lot of things that need to.

Userland/Libraries/LibWeb/DOM/DocumentLoadEventDelayer.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/DocumentLoadEventDelayer.h>
+
+namespace Web::DOM {
+
+DocumentLoadEventDelayer::DocumentLoadEventDelayer(Document& document)
+    : m_document(document)
+{
+    m_document->increment_number_of_things_delaying_the_load_event({});
+}
+
+DocumentLoadEventDelayer::~DocumentLoadEventDelayer()
+{
+    m_document->decrement_number_of_things_delaying_the_load_event({});
+}
+
+}