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/AbortSignal.cpp

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/AbortSignalWrapper.h>
+#include <LibWeb/DOM/AbortSignal.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/EventDispatcher.h>
+
+namespace Web::DOM {
+
+AbortSignal::AbortSignal(Document& document)
+    : EventTarget(static_cast<Bindings::ScriptExecutionContext&>(document))
+{
+}
+
+AbortSignal::~AbortSignal()
+{
+}
+
+bool AbortSignal::dispatch_event(NonnullRefPtr<Event> event)
+{
+    return EventDispatcher::dispatch(*this, event);
+}
+
+JS::Object* AbortSignal::create_wrapper(JS::GlobalObject& global_object)
+{
+    return wrap(global_object, *this);
+}
+
+// https://dom.spec.whatwg.org/#abortsignal-add
+void AbortSignal::add_abort_algorithm(Function<void()> abort_algorithm)
+{
+    if (m_aborted)
+        return;
+
+    m_abort_algorithms.append(move(abort_algorithm));
+}
+
+// https://dom.spec.whatwg.org/#abortsignal-signal-abort
+void AbortSignal::signal_abort()
+{
+    if (m_aborted)
+        return;
+
+    m_aborted = true;
+
+    for (auto& algorithm : m_abort_algorithms)
+        algorithm();
+
+    m_abort_algorithms.clear();
+
+    dispatch_event(Event::create(HTML::EventNames::abort));
+}
+
+}