Fill in descriptions on relevant blink_perf tests

PerformanceTests/Animation/overlay-background-color-transitions.html

 <!--
 Copyright (c) 2012 Cameron Adams. All rights reserved.
 Copyright (c) 2012 Code Aurora Forum. All rights reserved.
 Copyright (C) 2013 Google Inc. All rights reserved.
 Copyright (C) 2013 Intel Inc. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
 met:
     * Redistributions of source code must retain the above copyright
@@ skipping 50 matching lines @@
 <script src="resources/framerate.js"></script>
 <script>
 var maxOverlays = 500;
 var colors = ["rgba(204, 0, 0, 0.5)", "rgba(255, 204, 0, 0.5)", "rgba(170, 255, 0, 0.5)"
               , "rgba(0, 153, 204, 0.5)", "rgba(25, 76, 153, 0.5)", "rgba(102, 25, 153, 0.5)"];
 
 var testRunning = true;
 var overlays = [];
 
 window.onload = function () {
-    PerfTestRunner.prepareToMeasureValuesAsync({done: onCompletedRun, unit: 'fps'});
+    PerfTestRunner.prepareToMeasureValuesAsync({
+        description: "Measure performance of CSS Transitions animating background-color on many overlaid elements (only one element showing).",
+        done: onCompletedRun,
+        unit: 'fps'
+    });
 
     // Create the overlays
     for (var i = 0; i < maxOverlays; i++) {
         var overlay = new Overlay();
         overlay.start();
         overlays.push(overlay);
     }
 
     startTrackingFrameRate();
 }
@@ skipping 32 matching lines @@
 function onCompletedRun() {
     stopTrackingFrameRate();
 
     for (var i = 0; i < overlays.length; i++)
         overlays[i].destroy();
     overlays = [];
 }
 </script>
 </head>
 </html>