Fill in descriptions on relevant blink_perf tests

PerformanceTests/Animation/api-set-current-time-balls-keyframe-animations.html

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@@ 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: 'Measures performance of CSS keyframes animation using the web animations API and requestAnimationFrame (animating on left and top).',

[rjwright, 2014/10/15 03:37:12]

This test is a bit weird. It creates an animation and immediately pauses it,
then advances the animation by setting its current time inside a RAF call. I'd
describe it as


"Measure performance of animating 'left' and 'top' by calling Element.animate()
(from the Web Animations API), pausing the player, and updating the player's
startTime using requestAnimationFrame."

+        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 97 matching lines @@
     stopTrackingFrameRate();
 
     for (var i = 0; i < particles.length; i++)
         particles[i].destroy();
     particles = [];
 }
 </script>
 <style id="keyframes"></style>
 </head>
 </html>