[LayoutNG] Initial implementation of NGBoxIterator

third_party/WebKit/Source/core/layout/ng/ng_box_iterator.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/layout/ng/ng_box_iterator.h"
+#include "core/layout/ng/ng_box.h"
+
+namespace blink {
+
+NGBoxIterator::NGBoxIterator(const LayoutObject* layoutObject)
+    : m_layoutObject(layoutObject), m_currentChild(nullptr) {}
+
+NGBox NGBoxIterator::first() {
+  reset();
+  return next();
+}
+
+NGBox NGBoxIterator::next() {
+  if (!m_currentChild) {
+    m_currentChild = m_layoutObject->slowFirstChild();
+  } else {
+    m_currentChild = m_layoutObject->nextSibling();
+  }
+  return NGBox(m_currentChild);
+}
+
+void NGBoxIterator::reset() {
+  m_currentChild = nullptr;
+}
+
+}  // namespace blink