[CSS Grid Layout] Avoid inserting grid items twice in the grid

Source/core/rendering/RenderGrid.cpp

 /*
  * Copyright (C) 2011 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 184 matching lines @@
     // All of our children must be block level.
     setChildrenInline(false);
 }
 
 RenderGrid::~RenderGrid()
 {
 }
 
 void RenderGrid::addChild(RenderObject* newChild, RenderObject* beforeChild)
 {
+    // If the new requested beforeChild is not one of our children is because it's wrapped by an anonymous container. If
+    // we do not special case this situation we could end up calling addChild() twice for the newChild, one with the
+    // initial beforeChild and another one with its parent.
+    if (beforeChild && beforeChild->parent() != this) {
+        ASSERT(beforeChild->parent()->isAnonymous());
+        beforeChild = splitAnonymousBoxesAroundChild(beforeChild);
+    }
+
     RenderBlock::addChild(newChild, beforeChild);
 
     if (gridIsDirty())
         return;
 
     if (!newChild->isBox()) {
         dirtyGrid();
         return;
     }
 
@@ skipping 554 matching lines @@
 
 void RenderGrid::insertItemIntoGrid(RenderBox* child, const GridCoordinate& coordinate)
 {
     ensureGridSize(coordinate.rows.resolvedFinalPosition.toInt(), coordinate.columns.resolvedFinalPosition.toInt());
 
     for (GridSpan::iterator row = coordinate.rows.begin(); row != coordinate.rows.end(); ++row) {
         for (GridSpan::iterator column = coordinate.columns.begin(); column != coordinate.columns.end(); ++column)
             m_grid[row.toInt()][column.toInt()].append(child);
     }
 
+    RELEASE_ASSERT(!m_gridItemCoordinate.contains(child));
     m_gridItemCoordinate.set(child, coordinate);
 }
 
 void RenderGrid::placeItemsOnGrid()
 {
     if (!gridIsDirty())
         return;
 
     ASSERT(m_gridItemCoordinate.isEmpty());
 
@@ skipping 460 matching lines @@
     if (isOutOfFlowPositioned())
         return "RenderGrid (positioned)";
     if (isAnonymous())
         return "RenderGrid (generated)";
     if (isRelPositioned())
         return "RenderGrid (relative positioned)";
     return "RenderGrid";
 }
 
 } // namespace WebCore