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

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

 .grid {
     display: grid;
     background-color: grey;
 }
 
 .indefiniteSizeGrid {
     display: grid;
     background-color: grey;
     width: -webkit-min-content;
     height: auto;
@@ skipping 104 matching lines @@
     grid-column: 2;
     grid-row: auto;
 }
 
 .autoRowThirdColumn {
     background-color: magenta;
     grid-column: 3;
     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;
 }
 
 .gridAutoFlowColumn {
     grid-auto-flow: column;
 }
 
+.gridAutoFlowColumnDense {
+    grid-auto-flow: column dense;
+}
+
 .gridAutoFlowRow {

[Julien - ping for review, 2014/07/14 16:22:39]

I would put a FIXME to rename that to gridAutoFlowRowSparse to make it obvious
we are using the sparse algorithm.

[Manuel Rego, 2014/07/14 20:10:03]

I agree, I'll add the FIXME and provide a patch to do the rename.

     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;
     height: 10px;
 }
 
 .unconstrainedContainer {
     width: 1000px;
     height: 1000px;
 }
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     -webkit-writing-mode: vertical-lr;
 }
 
 .horizontalBT {
     -webkit-writing-mode: horizontal-bt;
 }
 
 .directionRTL {
     direction: rtl;
 }