LibWeb: Implement basic support for MessageChannel and MessagePort

This patch adds a basic initial implementation of these API's.

Since LibWeb currently doesn't support workers, this implementation of
messaging doesn't bother with serializing and deserializing messages.

Userland/Libraries/LibWeb/HTML/MessageChannel.cpp

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/HTML/MessageChannel.h>
+#include <LibWeb/HTML/MessagePort.h>
+
+namespace Web::HTML {
+
+MessageChannel::MessageChannel(Bindings::WindowObject& global_object)
+{
+    auto& context = global_object.impl().associated_document();
+
+    // 1. Set this's port 1 to a new MessagePort in this's relevant Realm.
+    m_port1 = MessagePort::create(context);
+
+    // 2. Set this's port 2 to a new MessagePort in this's relevant Realm.
+    m_port2 = MessagePort::create(context);
+
+    // 3. Entangle this's port 1 and this's port 2.
+    m_port1->entangle_with({}, *m_port2);
+}
+
+MessageChannel::~MessageChannel()
+{
+}
+
+}