Fill in descriptions on relevant blink_perf tests

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

Index: PerformanceTests/Animation/api-balls-keyframe-animations.html
diff --git a/PerformanceTests/Animation/api-balls-keyframe-animations.html b/PerformanceTests/Animation/api-balls-keyframe-animations.html
index f346116b552b1a58377a5b9522c99678d599626f..34ef44d1b48ec72814299b3a5dd0045b81981f9d 100644
--- a/PerformanceTests/Animation/api-balls-keyframe-animations.html
+++ b/PerformanceTests/Animation/api-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 (animating on left and top).',

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

These aren't actually CSS keyframes. Web Animations keyframes are just JS dicts.
Obviously they get turned into CSS style declarations, but I think a better
description would be something like

"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++) {