Use pass by reference in GridItemsSorter

Source/core/rendering/RenderGrid.cpp

Index: Source/core/rendering/RenderGrid.cpp
diff --git a/Source/core/rendering/RenderGrid.cpp b/Source/core/rendering/RenderGrid.cpp
index 3b1df259960c51148222fd24f532d74de36f09f2..fe73e5039c2415f9cf8000aee3d4e7fc63c986ad 100644
--- a/Source/core/rendering/RenderGrid.cpp
+++ b/Source/core/rendering/RenderGrid.cpp
@@ -1321,7 +1321,7 @@ static GridSpan dirtiedGridAreas(const Vector<LayoutUnit>& coordinates, LayoutUn
 
 class GridItemsSorter {
 public:
-    bool operator()(const std::pair<RenderBox*, size_t> firstChild, const std::pair<RenderBox*, size_t> secondChild) const
+    bool operator()(const std::pair<RenderBox*, size_t>& firstChild, const std::pair<RenderBox*, size_t>& secondChild) const
     {
         if (firstChild.first->style()->order() != secondChild.first->style()->order())
             return firstChild.first->style()->order() < secondChild.first->style()->order();