[css-grid] Use order-modified document order for m_gridItemsIndexesMap

We were storing the index in the DOM for the grid items
in m_gridItemsIndexesMap. However, we can already store the index
following the order-modified document order.

This allows us to simplify the sorter in painting,
as we don't need to check the order property now,
just simply use the stored index.

No new tests, as it's already covered by current tests.

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://chromiumcodereview.appspot.com/2438253003
Cr-Commit-Position: refs/heads/master@{#426989}

third_party/WebKit/Source/core/layout/LayoutGrid.cpp

@@ -1928,7 +1928,6 @@ void LayoutGrid::placeItemsOnGrid() {
     return;
 
   DCHECK(m_gridItemArea.isEmpty());
-  DCHECK(m_gridItemsIndexesMap.isEmpty());
 
   populateExplicitGridAndOrderIterator();
 
@@ -1937,12 +1936,16 @@ void LayoutGrid::placeItemsOnGrid() {
 
   Vector<LayoutBox*> autoMajorAxisAutoGridItems;
   Vector<LayoutBox*> specifiedMajorAxisAutoGridItems;
+  DCHECK(m_gridItemsIndexesMap.isEmpty());
+  size_t childIndex = 0;
   m_hasAnyOrthogonalChildren = false;
   for (LayoutBox* child = m_orderIterator.first(); child;
        child = m_orderIterator.next()) {
     if (child->isOutOfFlowPositioned())
       continue;
 
+    m_gridItemsIndexesMap.set(child, childIndex++);
+
     m_hasAnyOrthogonalChildren =
         m_hasAnyOrthogonalChildren || isOrthogonalChild(*child);
 
@@ -2003,12 +2006,9 @@ void LayoutGrid::populateExplicitGridAndOrderIterator() {
   size_t maximumColumnIndex = GridPositionsResolver::explicitGridColumnCount(
       *style(), m_autoRepeatColumns);
 
-  ASSERT(m_gridItemsIndexesMap.isEmpty());
-  size_t childIndex = 0;
   for (LayoutBox* child = firstInFlowChildBox(); child;
        child = child->nextInFlowSiblingBox()) {
     populator.collectChild(child);
-    m_gridItemsIndexesMap.set(child, childIndex++);
 
     // This function bypasses the cache (cachedGridArea()) as it is used to
     // build it.

third_party/WebKit/Source/core/paint/GridPainter.cpp

@@ -36,18 +36,13 @@ static GridSpan dirtiedGridAreas(const Vector<LayoutUnit>& coordinates,
                                               endGridAreaIndex + 1);
 }
 
-class GridItemsSorter {
- public:
-  bool operator()(const std::pair<LayoutBox*, size_t>& firstChild,
-                  const std::pair<LayoutBox*, size_t>& secondChild) const {
-    if (firstChild.first->style()->order() !=
-        secondChild.first->style()->order())
-      return firstChild.first->style()->order() <
-             secondChild.first->style()->order();
-
-    return firstChild.second < secondChild.second;
-  }
-};
+// Helper for the sorting of grid items following order-modified document order.
+// See http://www.w3.org/TR/css-flexbox/#order-modified-document-order
+static inline bool compareOrderModifiedDocumentOrder(
+    const std::pair<LayoutBox*, size_t>& firstItem,
+    const std::pair<LayoutBox*, size_t>& secondItem) {
+  return firstItem.second < secondItem.second;
+}
 
 void GridPainter::paintChildren(const PaintInfo& paintInfo,
                                 const LayoutPoint& paintOffset) {
@@ -103,10 +98,8 @@ void GridPainter::paintChildren(const PaintInfo& paintInfo,
           std::make_pair(item, m_layoutGrid.paintIndexForGridItem(item)));
   }
 
-  // Sort grid items following order-modified document order.
-  // See http://www.w3.org/TR/css-flexbox/#order-modified-document-order
   std::stable_sort(gridItemsToBePainted.begin(), gridItemsToBePainted.end(),
-                   GridItemsSorter());
+                   compareOrderModifiedDocumentOrder);
 
   LayoutBox* previous = 0;
   for (const auto& gridItemAndPaintIndex : gridItemsToBePainted) {