[CSS Grid Layout] Speed up painting on large grids

Source/core/rendering/OrderIterator.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 44 matching lines @@
             if (m_orderValuesIterator == m_orderValues.end())
                 return 0;
             if (m_orderValuesIterator) {
                 ++m_orderValuesIterator;
                 if (m_orderValuesIterator == m_orderValues.end())
                     return 0;
             } else {
                 m_orderValuesIterator = m_orderValues.begin();
             }
 
-            m_currentChild = m_containerBox->firstChildBox();
+            m_currentChild = firstChildBox();
         } else {
-            m_currentChild = m_currentChild->nextSiblingBox();
+            m_currentChild = nextSiblingBox();
         }
     } while (!m_currentChild || m_currentChild->style()->order() != *m_orderValuesIterator);
 
     return m_currentChild;
 }
 
 void OrderIterator::reset()
 {
     m_currentChild = 0;
     m_orderValuesIterator = 0;
 }
 
+RenderBox* OrderIterator::firstChildBox()
+{
+    if (m_children.isEmpty())
+        return m_containerBox->firstChildBox();
+
+    m_childIndex = 0;
+    return m_children[0];
+}
+
+RenderBox* OrderIterator::nextSiblingBox()
+{
+    if (m_children.isEmpty())
+        return m_currentChild->nextSiblingBox();
+
+    if (m_childIndex >= m_children.size() - 1)
+        return 0;
+
+    return m_children[++m_childIndex];
+}
+
 OrderIteratorPopulator::~OrderIteratorPopulator()
 {
     m_iterator.reset();
 
     if (m_anyChildHasDefaultOrderValue)
         m_iterator.m_orderValues.append(0);
 
     if (m_iterator.m_orderValues.size() > 1)
         removeDuplicatedOrderValues();
 
@@ skipping 13 matching lines @@
         int current = orderValues[i];
         if (current == previous)
             continue;
         ++uniqueItemIndex;
         std::swap(orderValues[i], orderValues[uniqueItemIndex]);
         previous = current;
     }
     orderValues.shrink(uniqueItemIndex + 1);
 }
 
+void OrderIteratorPopulator::storeChild(RenderBox* child)
+{
+    m_iterator.m_children.append(child);
+
+    collectChild(child);
+}
+
 void OrderIteratorPopulator::collectChild(const RenderBox* child)
 {
     // Avoid growing the vector for the common-case default value of 0.
     if (int order = child->style()->order())
         m_iterator.m_orderValues.append(order);
     else
         m_anyChildHasDefaultOrderValue = true;
 }
 
 } // namespace WebCore