Fill in descriptions on relevant blink_perf tests

PerformanceTests/Bindings/typed-array-construct-from-array.html

Index: PerformanceTests/Bindings/typed-array-construct-from-array.html
diff --git a/PerformanceTests/Bindings/typed-array-construct-from-array.html b/PerformanceTests/Bindings/typed-array-construct-from-array.html
index 4486dafc359632a9fd7de9269ef003e0e560d15e..436522735971c7824e7abb0ebb4ff29ec0cdfd79 100644
--- a/PerformanceTests/Bindings/typed-array-construct-from-array.html
+++ b/PerformanceTests/Bindings/typed-array-construct-from-array.html
@@ -8,9 +8,11 @@ var source = new Array(length);
 for (var i = 0; i < length; i++)
     source[i] = i;
 
-PerfTestRunner.measureRunsPerSecond({run: function() {
-    var target = new Uint8Array(source);
-}});
+PerfTestRunner.measureRunsPerSecond({
+    description: "Measures performance of creating a Uint8Array from an existing one million element Array which contains integers beyond the range of an unsigned 8-bit integer.",
+    run: function() {
+        var target = new Uint8Array(source);
+    }
+});
 </script>
 </body>
-