[CSS Grid Layout] Support span in auto-placement algorithm

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 1e3421ec370b71ad60d301a4eea450a521b788e8..fd675eed711d3321b2d49b6316dd90b913493413 100644
--- a/Source/core/rendering/style/GridCoordinate.h
+++ b/Source/core/rendering/style/GridCoordinate.h
@@ -114,6 +114,11 @@ struct GridSpan {
         return initialPositionIndex == o.initialPositionIndex && finalPositionIndex == o.finalPositionIndex;
     }
 
+    size_t spanningPositions() const

[Julien - ping for review, 2014/04/04 00:10:16]

This name is also confusing for me. I would rather to name it integerSpan (or
just "span" even if that would be ambiguous).

+    {
+        return finalPositionIndex - initialPositionIndex + 1;
+    }
+
     size_t initialPositionIndex;
     size_t finalPositionIndex;
 };