Fill in descriptions on relevant blink_perf tests

PerformanceTests/Animation/balls-composited-transform.html

Index: PerformanceTests/Animation/balls-composited-transform.html
diff --git a/PerformanceTests/Animation/balls-composited-transform.html b/PerformanceTests/Animation/balls-composited-transform.html
index c58c0831281263ee9f685568c4430f9b56793b17..4f0b1235f298520346efd4bba8fd11685cd83eac 100644
--- a/PerformanceTests/Animation/balls-composited-transform.html
+++ b/PerformanceTests/Animation/balls-composited-transform.html
@@ -70,7 +70,11 @@ var animationDuration = 10;
 var particles = [];
 
 window.onload = function () {
-    PerfTestRunner.prepareToMeasureValuesAsync({done: onCompletedRun, unit: 'fps'});
+    PerfTestRunner.prepareToMeasureValuesAsync({
+        description: 'Measures performance of CSS keyframes animation (animating on translate).',

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

"Measure performance of CSS Animations animating 'transform: translate'."

In this case the property being animated is important, and we should avoid
creating confusion about which API is being used.

+        done: onCompletedRun,
+        unit: 'fps'
+    });
 
     generateStyleElement();