Fill in descriptions on relevant blink_perf tests

PerformanceTests/Animation/balls-javascript.html

Index: PerformanceTests/Animation/balls-javascript.html
diff --git a/PerformanceTests/Animation/balls-javascript.html b/PerformanceTests/Animation/balls-javascript.html
index 4d7a5421ce0c1a7c757a3a396b76f81cf69db920..fac92361ececd3d9874d11a49362779e61d242f1 100644
--- a/PerformanceTests/Animation/balls-javascript.html
+++ b/PerformanceTests/Animation/balls-javascript.html
@@ -71,7 +71,11 @@ var particles = [];
 var prevTime;
 
 window.onload = function () {
-    PerfTestRunner.prepareToMeasureValuesAsync({done: onCompletedRun, unit: 'fps'});
+    PerfTestRunner.prepareToMeasureValuesAsync({
+        description: 'Measures animation performance of javascript to manipulate DOM elements (animating on left and top).',

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

Maybe this would be better described as

"Measure performance of animating 'left' and 'top' by updating inline styles in
requestAnimationFrame"

+        done: onCompletedRun,
+        unit: 'fps'
+    });
 
     // Create the particles
     for (var i = 0; i < maxParticles; i++)