Fill in descriptions on relevant blink_perf tests

PerformanceTests/Layout/fixed-grid-lots-of-data.html

Index: PerformanceTests/Layout/fixed-grid-lots-of-data.html
diff --git a/PerformanceTests/Layout/fixed-grid-lots-of-data.html b/PerformanceTests/Layout/fixed-grid-lots-of-data.html
index 8f6940769f58f9c63909057d5218a5f556dd11e4..5e0c53192ecc1efde98c33c5891ff5a045eaabe2 100644
--- a/PerformanceTests/Layout/fixed-grid-lots-of-data.html
+++ b/PerformanceTests/Layout/fixed-grid-lots-of-data.html
@@ -24,10 +24,13 @@ function startTest() {
     document.body.offsetHeight;
 
     var index = 0;
-    PerfTestRunner.measureRunsPerSecond({run: function() {
-        document.body.style.width = ++index % 2 ? "99%" : "98%";
-        document.body.offsetHeight;
-    }});
+    PerfTestRunner.measureRunsPerSecond({
+        description: "Measures performance of layout on a page using CSS grid layout (fixed).",
+        run: function() {
+            document.body.style.width = ++index % 2 ? "99%" : "98%";
+            document.body.offsetHeight;
+        }
+    });
 }
 </script>
 </head>