[CSS Grid Layout] Add GridSpan::iterator

Source/core/rendering/style/GridCoordinate.h

Index: Source/core/rendering/style/GridCoordinate.h
diff --git a/Source/core/rendering/style/GridCoordinate.h b/Source/core/rendering/style/GridCoordinate.h
index 126b88382568869e06175a0b13cdce859c03c7cc..b64eea45ed46f38899b46740819812af37660fb0 100644
--- a/Source/core/rendering/style/GridCoordinate.h
+++ b/Source/core/rendering/style/GridCoordinate.h
@@ -117,6 +117,18 @@ struct GridSpan {
 
     GridResolvedPosition resolvedInitialPosition;
     GridResolvedPosition resolvedFinalPosition;
+
+    typedef GridResolvedPosition iterator;
+
+    iterator begin() const
+    {
+        return resolvedInitialPosition;
+    }
+
+    iterator end() const
+    {
+        return resolvedFinalPosition.next();
+    }
 };
 
 // This represents a grid area that spans in both rows' and columns' direction.