[CSS Grid Layout] Remove stack from grid-auto-flow syntax

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 159 matching lines @@
     grid-row: span 2;
 }
 
 .autoRowSpanning3AutoColumnSpanning2 {
     background-color: indigo;
     grid-column: span 2;
     grid-row: span 3;
 }
 
 /* Grid element flow. */
-.gridAutoFlowStack {
-    grid-auto-flow: stack;
-}
-
 .gridAutoFlowColumnSparse {
     grid-auto-flow: column;
 }
 
 .gridAutoFlowColumnDense {
     grid-auto-flow: column dense;
 }
 
 .gridAutoFlowRowSparse {
     grid-auto-flow: row;
@@ skipping 33 matching lines @@
     -webkit-writing-mode: horizontal-bt;
 }
 
 .directionRTL {
     direction: rtl;
 }
 
 .directionLTR {
     direction: ltr;
 }