LibWeb+Meta: Add wrapper for the BufferSource/ArrayBufferView IDL types

These wrappers will make it much easier to do various operations on the
different ArrayBuffer-related classes in LibWeb compared to the current
solution, which is to just accept a Handle<Object> everywhere (and use
"any" in the *.idl files).

Co-Authored-By: Matthew Olsson <mattco@serenityos.org>

Userland/Libraries/LibWeb/WebIDL/Buffers.cpp

@@ -0,0 +1,82 @@
+/*
+ * Copyright (c) 2023, Shannon Booth <shannon@serenityos.org>
+ * Copyright (c) 2023, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/DataView.h>
+#include <LibJS/Runtime/TypedArray.h>
+#include <LibWeb/WebIDL/Buffers.h>
+
+namespace Web::WebIDL {
+
+u32 BufferableObjectBase::byte_length() const
+{
+    return m_bufferable_object.visit([](auto& obj) { return static_cast<u32>(obj->byte_length()); });
+}
+
+JS::NonnullGCPtr<JS::Object> BufferableObjectBase::raw_object()
+{
+    return m_bufferable_object.visit([](auto const& obj) -> JS::NonnullGCPtr<JS::Object> { return obj; });
+}
+
+JS::GCPtr<JS::ArrayBuffer> BufferableObjectBase::viewed_array_buffer()
+{
+    return m_bufferable_object.visit(
+        [](JS::NonnullGCPtr<JS::ArrayBuffer> array_buffer) -> JS::GCPtr<JS::ArrayBuffer> { return array_buffer; },
+        [](auto const& view) -> JS::GCPtr<JS::ArrayBuffer> { return view->viewed_array_buffer(); });
+}
+
+BufferableObject BufferableObjectBase::bufferable_object_from_raw_object(JS::NonnullGCPtr<JS::Object> object)
+{
+    if (is<JS::TypedArrayBase>(*object))
+        return JS::NonnullGCPtr { static_cast<JS::TypedArrayBase&>(*object) };
+
+    if (is<JS::DataView>(*object))
+        return JS::NonnullGCPtr { static_cast<JS::DataView&>(*object) };
+
+    if (is<JS::ArrayBuffer>(*object))
+        return JS::NonnullGCPtr { static_cast<JS::ArrayBuffer&>(*object) };
+
+    VERIFY_NOT_REACHED();
+}
+
+BufferableObjectBase::BufferableObjectBase(JS::NonnullGCPtr<JS::Object> object)
+    : m_bufferable_object(bufferable_object_from_raw_object(object))
+{
+}
+
+bool BufferableObjectBase::is_typed_array_base() const
+{
+    return m_bufferable_object.has<JS::NonnullGCPtr<JS::TypedArrayBase>>();
+}
+
+bool BufferableObjectBase::is_data_view() const
+{
+    return m_bufferable_object.has<JS::NonnullGCPtr<JS::DataView>>();
+}
+
+bool BufferableObjectBase::is_array_buffer() const
+{
+    return m_bufferable_object.has<JS::NonnullGCPtr<JS::ArrayBuffer>>();
+}
+
+void BufferableObjectBase::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    m_bufferable_object.visit([&](auto& obj) { visitor.visit(obj); });
+}
+
+ArrayBufferView::~ArrayBufferView() = default;
+
+u32 ArrayBufferView::byte_offset() const
+{
+    return m_bufferable_object.visit(
+        [](JS::NonnullGCPtr<JS::ArrayBuffer>) -> u32 { VERIFY_NOT_REACHED(); },
+        [](auto& view) -> u32 { return static_cast<u32>(view->byte_offset()); });
+}
+
+BufferSource::~BufferSource() = default;
+
+}