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..a9237e4bf870f00c07d5cf09cae026d9a8b0d0b3 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: '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++) {