LibWeb: Implement basic version of CSSOM View's VisualViewport

We got some errors while loading https://twinings.co.uk/ about this
interface missing, and it looked fairly simple so I sketched it out.
Note that I did leave some FIXMEs where it's not clear exactly which
metrics we should be returning.

Userland/Libraries/LibWeb/CSS/VisualViewport.cpp

@@ -0,0 +1,154 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/Bindings/VisualViewportPrototype.h>
+#include <LibWeb/CSS/VisualViewport.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/EventDispatcher.h>
+#include <LibWeb/DOM/IDLEventListener.h>
+#include <LibWeb/HTML/EventHandler.h>
+#include <LibWeb/HTML/EventNames.h>
+
+namespace Web::CSS {
+
+WebIDL::ExceptionOr<JS::NonnullGCPtr<VisualViewport>> VisualViewport::create(DOM::Document& document)
+{
+    return MUST_OR_THROW_OOM(document.heap().allocate<VisualViewport>(document.realm(), document));
+}
+
+VisualViewport::VisualViewport(DOM::Document& document)
+    : DOM::EventTarget(document.realm())
+    , m_document(document)
+{
+}
+
+JS::ThrowCompletionOr<void> VisualViewport::initialize(JS::Realm& realm)
+{
+    MUST_OR_THROW_OOM(Base::initialize(realm));
+    set_prototype(&Bindings::ensure_web_prototype<Bindings::VisualViewportPrototype>(realm, "VisualViewport"));
+
+    return {};
+}
+
+void VisualViewport::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_document);
+}
+
+// https://drafts.csswg.org/cssom-view/#dom-visualviewport-offsetleft
+double VisualViewport::offset_left() const
+{
+    // 1. If the visual viewport’s associated document is not fully active, return 0.
+    if (!m_document->is_fully_active())
+        return 0;
+
+    // 2. Otherwise, return the offset of the left edge of the visual viewport from the left edge of the layout viewport.
+    VERIFY(m_document->browsing_context());
+    return m_document->browsing_context()->viewport_rect().left().to_double();
+}
+
+// https://drafts.csswg.org/cssom-view/#dom-visualviewport-offsettop
+double VisualViewport::offset_top() const
+{
+    // 1. If the visual viewport’s associated document is not fully active, return 0.
+    if (!m_document->is_fully_active())
+        return 0;
+
+    // 2. Otherwise, return the offset of the top edge of the visual viewport from the top edge of the layout viewport.
+    VERIFY(m_document->browsing_context());
+    return m_document->browsing_context()->viewport_rect().top().to_double();
+}
+
+// https://drafts.csswg.org/cssom-view/#dom-visualviewport-pageleft
+double VisualViewport::page_left() const
+{
+    // 1. If the visual viewport’s associated document is not fully active, return 0.
+    if (!m_document->is_fully_active())
+        return 0;
+
+    // FIXME: 2. Otherwise, return the offset of the left edge of the visual viewport
+    //           from the left edge of the initial containing block of the layout viewport’s document.
+    return offset_left();
+}
+
+// https://drafts.csswg.org/cssom-view/#dom-visualviewport-pagetop
+double VisualViewport::page_top() const
+{
+    // 1. If the visual viewport’s associated document is not fully active, return 0.
+    if (!m_document->is_fully_active())
+        return 0;
+
+    // FIXME: 2. Otherwise, return the offset of the top edge of the visual viewport
+    //           from the top edge of the initial containing block of the layout viewport’s document.
+    return offset_top();
+}
+
+// https://drafts.csswg.org/cssom-view/#dom-visualviewport-width
+double VisualViewport::width() const
+{
+    // 1. If the visual viewport’s associated document is not fully active, return 0.
+    if (!m_document->is_fully_active())
+        return 0;
+
+    // 2. Otherwise, return the width of the visual viewport
+    //    FIXME: excluding the width of any rendered vertical classic scrollbar that is fixed to the visual viewport.
+    VERIFY(m_document->browsing_context());
+    return m_document->browsing_context()->viewport_rect().width().to_double();
+}
+
+// https://drafts.csswg.org/cssom-view/#dom-visualviewport-height
+double VisualViewport::height() const
+{
+    // 1. If the visual viewport’s associated document is not fully active, return 0.
+    if (!m_document->is_fully_active())
+        return 0;
+
+    // 2. Otherwise, return the height of the visual viewport
+    //    FIXME: excluding the height of any rendered vertical classic scrollbar that is fixed to the visual viewport.
+    VERIFY(m_document->browsing_context());
+    return m_document->browsing_context()->viewport_rect().height().to_double();
+}
+
+// https://drafts.csswg.org/cssom-view/#dom-visualviewport-scale
+double VisualViewport::scale() const
+{
+    // FIXME: Implement.
+    return 1;
+}
+
+void VisualViewport::set_onresize(WebIDL::CallbackType* event_handler)
+{
+    set_event_handler_attribute(HTML::EventNames::resize, event_handler);
+}
+
+WebIDL::CallbackType* VisualViewport::onresize()
+{
+    return event_handler_attribute(HTML::EventNames::resize);
+}
+
+void VisualViewport::set_onscroll(WebIDL::CallbackType* event_handler)
+{
+    set_event_handler_attribute(HTML::EventNames::scroll, event_handler);
+}
+
+WebIDL::CallbackType* VisualViewport::onscroll()
+{
+    return event_handler_attribute(HTML::EventNames::scroll);
+}
+
+void VisualViewport::set_onscrollend(WebIDL::CallbackType* event_handler)
+{
+    set_event_handler_attribute(HTML::EventNames::scrollend, event_handler);
+}
+
+WebIDL::CallbackType* VisualViewport::onscrollend()
+{
+    return event_handler_attribute(HTML::EventNames::scrollend);
+}
+
+}