Fill in descriptions on relevant blink_perf tests

PerformanceTests/Animation/api-balls-keyframe-animations.html

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 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.
 
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 notice, this list of conditions and the following disclaimer.
@@ skipping 53 matching lines @@
 var minVelocity = 50;
 var maxVelocity = 500;
 var particleRadius = 6;
 var colors = ["#cc0000", "#ffcc00", "#aaff00", "#0099cc", "#194c99", "#661999"];
 var animationDuration = 10;
 
 var particles = [];
 var allParticleKeyframes = [];
 
 window.onload = function () {
-    PerfTestRunner.prepareToMeasureValuesAsync({done: onCompletedRun, unit: 'fps'});
+    PerfTestRunner.prepareToMeasureValuesAsync({
+        description: "Measure performance of Web Animations API animating 'left' and 'top' using Element.animate()",
+        done: onCompletedRun,
+        unit: 'fps'
+    });
 
     // Create the particles
     for (var i = 0; i < maxParticles; i++) {
         generateParticleKeyframes(i);
         var particle = new Particle(i);
         particle.start();
         particles.push(particle);
     }
 
     startTrackingFrameRate();
@@ skipping 80 matching lines @@
     stopTrackingFrameRate();
 
     for (var i = 0; i < particles.length; i++)
         particles[i].destroy();
     particles = [];
 }
 </script>
 <style id="keyframes"></style>
 </head>
 </html>