LibWeb: Stub out a basic ResizeObserver interface

This patch establishes scaffolding for the ResizeObserver API.

Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.cpp

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+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/ResizeObserver/ResizeObserver.h>
+
+namespace Web::ResizeObserver {
+
+// https://drafts.csswg.org/resize-observer/#dom-resizeobserver-resizeobserver
+NonnullRefPtr<ResizeObserver> ResizeObserver::create_with_global_object(JS::GlobalObject& global_object, JS::Value callback)
+{
+    // FIXME: Implement
+    (void)global_object;
+    (void)callback;
+    return adopt_ref(*new ResizeObserver);
+}
+
+// https://drafts.csswg.org/resize-observer/#dom-resizeobserver-observe
+void ResizeObserver::observe(DOM::Element& target, ResizeObserverOptions options)
+{
+    // FIXME: Implement
+    (void)target;
+    (void)options;
+}
+
+// https://drafts.csswg.org/resize-observer/#dom-resizeobserver-unobserve
+void ResizeObserver::unobserve(DOM::Element& target)
+{
+    // FIXME: Implement
+    (void)target;
+}
+
+// https://drafts.csswg.org/resize-observer/#dom-resizeobserver-disconnect
+void ResizeObserver::disconnect()
+{
+    // FIXME: Implement
+}
+
+}

Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.h

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+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+#include <LibWeb/Bindings/Wrappable.h>
+
+namespace Web::ResizeObserver {
+
+struct ResizeObserverOptions {
+    String box;
+};
+
+// https://drafts.csswg.org/resize-observer/#resize-observer-interface
+class ResizeObserver
+    : public RefCounted<ResizeObserver>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::ResizeObserverWrapper;
+
+    static NonnullRefPtr<ResizeObserver> create_with_global_object(JS::GlobalObject&, JS::Value callback);
+
+    void observe(DOM::Element& target, ResizeObserverOptions);
+    void unobserve(DOM::Element& target);
+    void disconnect();
+};
+
+}

Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.idl

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+[Exposed=(Window)]
+interface ResizeObserver {
+
+    // FIXME: This should be a ResizeObserverCallback.
+    constructor(any callback);
+    undefined observe(Element target, optional ResizeObserverOptions options = {});
+    undefined unobserve(Element target);
+    undefined disconnect();
+
+};
+
+dictionary ResizeObserverOptions {
+
+    // FIXME: This should be an enum.
+    DOMString box = "content-box";
+
+};