Add support for parsing <grid-line> that includes a 'span'

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 8877297f4938f11dc3e6d175b3eaaf86cba1b3f9..e6e07d1eccfaa20f8940ea6403f94567c9032c85 100644
--- a/Source/core/rendering/RenderGrid.cpp
+++ b/Source/core/rendering/RenderGrid.cpp
@@ -795,6 +795,10 @@ size_t RenderGrid::resolveGridPositionFromStyle(const GridPosition& position, Gr
         // 'auto' depends on the opposite position for resolution (e.g. grid-row: auto / 1).
         ASSERT_NOT_REACHED();
         return 0;
+    case SpanPosition:
+        // FIXME: Handle span positions (crbug.com/229713).
+        ASSERT_NOT_REACHED();
+        return 0;
     }
     ASSERT_NOT_REACHED();
     return 0;