[CSS Grid Layout] Unknown grid area should compute to 'auto'

LayoutTests/fast/css-grid-layout/grid-item-area-get-set.html

Index: LayoutTests/fast/css-grid-layout/grid-item-area-get-set.html
diff --git a/LayoutTests/fast/css-grid-layout/grid-item-area-get-set.html b/LayoutTests/fast/css-grid-layout/grid-item-area-get-set.html
index 2c9743858b23fa013fbf35071a911219d4a3361d..ef51808e979f6f4dfa987d7cfb5fc4850e5865d2 100644
--- a/LayoutTests/fast/css-grid-layout/grid-item-area-get-set.html
+++ b/LayoutTests/fast/css-grid-layout/grid-item-area-get-set.html
@@ -4,8 +4,8 @@
 <link href="resources/grid.css" rel="stylesheet">
 <style>
 .grid {
-    grid-template: "firstArea" "secondArea"
-                   "thirdArea" "thirdArea";
+    grid-template: "firstArea secondArea"
+                   "thirdArea thirdArea";
 }
 
 #oneValueGridArea {
@@ -129,7 +129,7 @@
     testGridAreaCSSParsing("twoValueIdentGridArea", "firstArea", "secondArea", "firstArea", "secondArea");
     testGridAreaCSSParsing("threeValueGridArea", "span 1", "10", "-1");
     testGridAreaCSSParsing("fourValueGridArea", "-5", "span 5", "span 8", "9");
-    testGridAreaCSSParsing("fourValueMixedGridArea", "firstArea", "span 1", "nonExistent", "1 foobar");
+    testGridAreaCSSParsing("fourValueMixedGridArea", "firstArea", "span 1", "auto", "1 foobar");
 
     debug("");
     debug("Test getting and setting grid-area set through JS");