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

LayoutTests/fast/css-grid-layout/resources/grid.css

Index: LayoutTests/fast/css-grid-layout/resources/grid.css
diff --git a/LayoutTests/fast/css-grid-layout/resources/grid.css b/LayoutTests/fast/css-grid-layout/resources/grid.css
index e28c00e070886043baffad16381db23b516b8122..ed2266ea74dbffada2b0275967ebfe9ab4a81296 100644
--- a/LayoutTests/fast/css-grid-layout/resources/grid.css
+++ b/LayoutTests/fast/css-grid-layout/resources/grid.css
@@ -122,19 +122,53 @@
     grid-row: auto;
 }
 
+.autoRowAutoColumnSpanning2 {
+    background-color: maroon;
+    grid-column: span 2;
+    grid-row: auto;
+}
+
+.autoRowSpanning2AutoColumn {
+    background-color: aqua;
+    grid-column: auto;
+    grid-row: span 2;
+}
+
+.autoRowSpanning2AutoColumnSpanning3 {
+    background-color: olive;
+    grid-column: span 3;
+    grid-row: span 2;
+}
+
+.autoRowSpanning3AutoColumnSpanning2 {
+    background-color: indigo;
+    grid-column: span 2;
+    grid-row: span 3;
+}
+
 /* Grid element flow. */
 .gridAutoFlowStack {
     grid-auto-flow: stack;
 }
 
+/* FIXME: Rename to gridAutoFlowColumnSparse to be more explicit about the algorithm mode by default. */
 .gridAutoFlowColumn {
     grid-auto-flow: column;
 }
 
+.gridAutoFlowColumnDense {
+    grid-auto-flow: column dense;
+}
+
+/* FIXME: Rename to gridAutoFlowRowSparse to be more explicit about the algorithm mode by default. */
 .gridAutoFlowRow {
     grid-auto-flow: row;
 }
 
+.gridAutoFlowRowDense {
+    grid-auto-flow: row dense;
+}
+
 /* This rule makes sure the container is smaller than any grid items to avoid distributing any extra logical space to them. */
 .constrainedContainer {
     width: 10px;