Fill in descriptions on relevant blink_perf tests

PerformanceTests/Animation/balls-composited-transform.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.
 
 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
 notice, this list of conditions and the following disclaimer.
@@ skipping 52 matching lines @@
 var maxParticles = 2500;
 var minVelocity = 50;
 var maxVelocity = 500;
 var particleRadius = 6;
 var colors = ["#cc0000", "#ffcc00", "#aaff00", "#0099cc", "#194c99", "#661999"];
 var animationDuration = 10;
 
 var particles = [];
 
 window.onload = function () {
-    PerfTestRunner.prepareToMeasureValuesAsync({done: onCompletedRun, unit: 'fps'});
+    PerfTestRunner.prepareToMeasureValuesAsync({
+        description: "Measure performance of CSS Animations animating 'transform: translate'.",
+        done: onCompletedRun,
+        unit: 'fps'
+    });
 
     generateStyleElement();
 
     // Create the particles
     for (var i = 0; i < maxParticles; i++) {
         var particle = new Particle(i);
         particle.start();
         particles.push(particle);
     }
 
@@ skipping 102 matching lines @@
     stopTrackingFrameRate();
 
     for (var i = 0; i < particles.length; i++)
         particles[i].destroy();
     particles = [];
 }
 </script>
 <style id="keyframes"></style>
 </head>
 </html>