Fix handling of 'inherit' and 'initial' for grid lines

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

Index: LayoutTests/fast/css-grid-layout/grid-item-end-after-get-set.html
diff --git a/LayoutTests/fast/css-grid-layout/grid-item-end-after-get-set.html b/LayoutTests/fast/css-grid-layout/grid-item-end-after-get-set.html
index 2baa7c19457188c175c42f543030996fdd233c08..968fc8e9e66a369b212dcb1742ced082ab772db4 100644
--- a/LayoutTests/fast/css-grid-layout/grid-item-end-after-get-set.html
+++ b/LayoutTests/fast/css-grid-layout/grid-item-end-after-get-set.html
@@ -72,6 +72,16 @@ if (window.testRunner)
     testEndAfterJSParsing("auto", "auto");
 
     debug("");
+    debug("Test setting grid-start and grid-before to 'inherit' through JS");
+    testEndAfterInheritJSParsing("inherit", "18");
+    testEndAfterInheritJSParsing("2", "inherit");
+    testEndAfterInheritJSParsing("inherit", "inherit");
+
+    debug("");
+    debug("Test setting grid-start and grid-before to 'initial' through JS");
+    testEndAfterInitialJSParsing();
+
+    debug("");
     debug("Test setting grid-end and grid-after back to 'auto' through JS");
     element.style.webkitGridEnd = "18";
     element.style.webkitGridAfter = "66";