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/MessagePort.h

@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <AK/Weakable.h>
+#include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/DOM/EventTarget.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::HTML {
+
+#define ENUMERATE_MESSAGE_PORT_EVENT_HANDLERS(E) \
+    E(onmessage, HTML::EventNames::message)      \
+    E(onmessageerror, HTML::EventNames::messageerror)
+
+// https://html.spec.whatwg.org/multipage/web-messaging.html#message-ports
+class MessagePort final
+    : public RefCounted<MessagePort>
+    , public Weakable<MessagePort>
+    , public DOM::EventTarget
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::MessagePortWrapper;
+
+    using RefCounted::ref;
+    using RefCounted::unref;
+
+    static NonnullRefPtr<MessagePort> create(Bindings::ScriptExecutionContext& context)
+    {
+        return adopt_ref(*new MessagePort(context));
+    }
+
+    virtual ~MessagePort() override;
+
+    // ^EventTarget
+    virtual void ref_event_target() override { ref(); }
+    virtual void unref_event_target() override { unref(); }
+    virtual bool dispatch_event(NonnullRefPtr<DOM::Event>) override;
+    virtual JS::Object* create_wrapper(JS::GlobalObject&) override;
+
+    // https://html.spec.whatwg.org/multipage/web-messaging.html#entangle
+    void entangle_with(Badge<MessageChannel>, MessagePort&);
+
+    // https://html.spec.whatwg.org/multipage/web-messaging.html#dom-messageport-postmessage
+    void post_message(JS::Value);
+
+    void start();
+
+    void close();
+
+#undef __ENUMERATE
+#define __ENUMERATE(attribute_name, event_name)    \
+    void set_##attribute_name(HTML::EventHandler); \
+    HTML::EventHandler attribute_name();
+    ENUMERATE_MESSAGE_PORT_EVENT_HANDLERS(__ENUMERATE)
+#undef __ENUMERATE
+
+private:
+    explicit MessagePort(Bindings::ScriptExecutionContext&);
+
+    bool is_entangled() const { return m_remote_port; }
+    void disentangle();
+
+    // The HTML spec implies(!) that this is MessagePort.[[RemotePort]]
+    WeakPtr<MessagePort> m_remote_port;
+
+    // https://html.spec.whatwg.org/multipage/web-messaging.html#has-been-shipped
+    bool m_has_been_shipped { false };
+
+    // This is TransferableObject.[[Detached]]
+    bool m_detached { false };
+};
+
+}