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

Source/core/rendering/RenderGrid.h

Index: Source/core/rendering/RenderGrid.h
diff --git a/Source/core/rendering/RenderGrid.h b/Source/core/rendering/RenderGrid.h
index b893585acb42ee9cb809edc90d27192e734927c3..ae88f4fc4df044dbd231f8b99f3d48aa4f929fb4 100644
--- a/Source/core/rendering/RenderGrid.h
+++ b/Source/core/rendering/RenderGrid.h
@@ -35,10 +35,7 @@ struct GridCoordinate;
 struct GridSpan;
 class GridTrack;
 
-enum GridTrackSizingDirection {
-    ForColumns,
-    ForRows
-};
+typedef HashMap<String, Vector<size_t> > NamedGridLinesMap;
 
 class RenderGrid FINAL : public RenderBlock {
 public:
@@ -124,6 +121,7 @@ private:
     GridSpan resolveGridPositionsFromAutoPlacementPosition(const RenderBox*, GridTrackSizingDirection, size_t) const;
     PassOwnPtr<GridSpan> resolveGridPositionsFromStyle(const RenderBox*, GridTrackSizingDirection) const;
     size_t resolveNamedGridLinePositionFromStyle(const GridPosition&, GridPositionSide) const;
+    size_t resolveExplicitPositionFromStyle(const GridPosition&, GridPositionSide) const;
     size_t resolveGridPositionFromStyle(const GridPosition&, GridPositionSide) const;
     PassOwnPtr<GridSpan> resolveGridPositionAgainstOppositePosition(size_t resolvedOppositePosition, const GridPosition&, GridPositionSide) const;
     PassOwnPtr<GridSpan> resolveNamedGridLinePositionAgainstOppositePosition(size_t resolvedOppositePosition, const GridPosition&, GridPositionSide) const;
@@ -151,6 +149,9 @@ private:
         return m_grid.size();
     }
 
+    const NamedGridLinesMap& gridLinesForSide(GridPositionSide side) const { return (side == ColumnStartSide || side == ColumnEndSide) ? m_namedGridColumnLines : m_namedGridRowLines; }
+    void createGridLineNamesMapsForDirection(GridTrackSizingDirection);
+
     typedef Vector<RenderBox*, 1> GridCell;
     typedef Vector<Vector<GridCell> > GridRepresentation;
     GridRepresentation m_grid;
@@ -161,6 +162,9 @@ private:
     OrderIterator m_orderIterator;
     Vector<RenderBox*> m_gridItemsOverflowingGridArea;
 
+    NamedGridLinesMap m_namedGridColumnLines;
+    NamedGridLinesMap m_namedGridRowLines;
+
     friend class GridCoordinateSorter;
 };