[CSS Grid Layout] Add support to place items using named grid lines

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 b64eea45ed46f38899b46740819812af37660fb0..4fb49abeafca59233ca16707fc61d36457d74485 100644
--- a/Source/core/rendering/style/GridCoordinate.h
+++ b/Source/core/rendering/style/GridCoordinate.h
@@ -156,6 +156,22 @@ struct GridCoordinate {
         return !(*this == o);
     }
 
+    GridResolvedPosition positionForSide(GridPositionSide side) const
+    {
+        switch (side) {
+        case ColumnStartSide:
+            return columns.resolvedInitialPosition;
+        case ColumnEndSide:
+            return columns.resolvedFinalPosition;
+        case RowStartSide:
+            return rows.resolvedInitialPosition;
+        case RowEndSide:
+            return rows.resolvedFinalPosition;
+        }
+        ASSERT_NOT_REACHED();
+        return 0;
+    }
+
     GridSpan columns;
     GridSpan rows;
 };