Fill in descriptions on relevant blink_perf tests

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

Index: PerformanceTests/Animation/api-set-current-time-balls-keyframe-animations.html
diff --git a/PerformanceTests/Animation/api-set-current-time-balls-keyframe-animations.html b/PerformanceTests/Animation/api-set-current-time-balls-keyframe-animations.html
index d98c6e3c5aa1bdbc22c683fc59d50736879d59ab..0b20cd08584503ff6c745df497765e4b7aa18c9d 100644
--- a/PerformanceTests/Animation/api-set-current-time-balls-keyframe-animations.html
+++ b/PerformanceTests/Animation/api-set-current-time-balls-keyframe-animations.html
@@ -71,7 +71,11 @@ var particles = [];
 var allParticleKeyframes = [];
 
 window.onload = function () {
-    PerfTestRunner.prepareToMeasureValuesAsync({done: onCompletedRun, unit: 'fps'});
+    PerfTestRunner.prepareToMeasureValuesAsync({
+        description: "Measure performance of animating 'left' and 'top' by calling Element.animate() (from the Web Animations API), pausing the player, and updating the player's currentTime using requestAnimationFrame.",
+        done: onCompletedRun,
+        unit: 'fps'
+    });
 
     // Create the particles
     for (var i = 0; i < maxParticles; i++) {