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

LayoutTests/fast/css-grid-layout/grid-item-auto-placement-definite-span.html

Index: LayoutTests/fast/css-grid-layout/grid-item-auto-placement-definite-span.html
diff --git a/LayoutTests/fast/css-grid-layout/grid-item-auto-placement-definite-span.html b/LayoutTests/fast/css-grid-layout/grid-item-auto-placement-definite-span.html
index 35e2ae1e7f537bce030c825628bf92f179ad90c8..a12db68be56af677087a8597062810f97b25cb9b 100644
--- a/LayoutTests/fast/css-grid-layout/grid-item-auto-placement-definite-span.html
+++ b/LayoutTests/fast/css-grid-layout/grid-item-auto-placement-definite-span.html
@@ -52,7 +52,7 @@
 <p>Thist test checks that span is supported in auto-placement for definite positions.</p>
 
 <div style="position: relative">
-    <div class="grid gridAutoFlowRow">
+    <div class="grid gridAutoFlowRowDense">
         <div class="sizedToGridArea autoRowFirstColumnSpanning2" data-offset-x="0" data-offset-y="0" data-expected-width="200" data-expected-height="50"></div>
         <div class="sizedToGridArea autoRowSecondColumnSpanning2" data-offset-x="100" data-offset-y="50" data-expected-width="200" data-expected-height="50"></div>
         <div class="sizedToGridArea autoRowFirstColumnSpanning3" data-offset-x="0" data-offset-y="100" data-expected-width="300" data-expected-height="50"></div>
@@ -72,7 +72,7 @@
 </div>
 
 <div style="position: relative">
-    <div class="grid gridAutoFlowColumn">
+    <div class="grid gridAutoFlowColumnDense">
         <div class="sizedToGridArea firstRowSpanning2AutoColumn" data-offset-x="0" data-offset-y="0" data-expected-width="100" data-expected-height="100"></div>
         <div class="sizedToGridArea secondRowSpanning2AutoColumn" data-offset-x="100" data-offset-y="50" data-expected-width="100" data-expected-height="100"></div>
         <div class="sizedToGridArea firstRowSpanning3AutoColumn" data-offset-x="200" data-offset-y="0" data-expected-width="100" data-expected-height="150"></div>