LibWeb: Add initial support for AbortController and AbortSignal

The DOM specification says that the primary use case for these is to
give Promises abort semantics. It is also a prerequisite for Fetch,
as it is used to make Fetch abortable.
a

Userland/Libraries/LibWeb/DOM/AbortController.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <AK/Weakable.h>
+#include <LibWeb/Bindings/WindowObject.h>
+#include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/DOM/AbortSignal.h>
+#include <LibWeb/DOM/Window.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::DOM {
+
+// https://dom.spec.whatwg.org/#abortcontroller
+class AbortController final
+    : public RefCounted<AbortController>
+    , public Weakable<AbortController>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::AbortControllerWrapper;
+
+    static NonnullRefPtr<AbortController> create(Document& document)
+    {
+        return adopt_ref(*new AbortController(document));
+    }
+
+    static NonnullRefPtr<AbortController> create_with_global_object(Bindings::WindowObject& window_object)
+    {
+        return AbortController::create(window_object.impl().document());
+    }
+
+    virtual ~AbortController() override;
+
+    // https://dom.spec.whatwg.org/#dom-abortcontroller-signal
+    NonnullRefPtr<AbortSignal> signal() const { return m_signal; }
+
+    void abort();
+
+private:
+    AbortController(Document& document);
+
+    // https://dom.spec.whatwg.org/#abortcontroller-signal
+    NonnullRefPtr<AbortSignal> m_signal;
+};
+
+}