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.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/AbortController.h>
+#include <LibWeb/DOM/AbortSignal.h>
+
+namespace Web::DOM {
+
+// https://dom.spec.whatwg.org/#dom-abortcontroller-abortcontroller
+AbortController::AbortController(Document& document)
+    : m_signal(AbortSignal::create(document))
+{
+}
+
+AbortController::~AbortController()
+{
+}
+
+// https://dom.spec.whatwg.org/#dom-abortcontroller-abort
+void AbortController::abort()
+{
+    m_signal->signal_abort();
+}
+
+}