Fill in descriptions on relevant blink_perf tests

PerformanceTests/Animation/balls-transition-20-properties.html

Index: PerformanceTests/Animation/balls-transition-20-properties.html
diff --git a/PerformanceTests/Animation/balls-transition-20-properties.html b/PerformanceTests/Animation/balls-transition-20-properties.html
index 174d32798d521289b7bb2b957d83b4a57434c5f7..2236ded59a8c018cdca080236755340d3afd2214 100644
--- a/PerformanceTests/Animation/balls-transition-20-properties.html
+++ b/PerformanceTests/Animation/balls-transition-20-properties.html
@@ -70,7 +70,11 @@ var testRunning = true;
 var particles = [];
 
 window.onload = function () {
-    PerfTestRunner.prepareToMeasureValuesAsync({done: onCompletedRun, unit: 'fps'});
+    PerfTestRunner.prepareToMeasureValuesAsync({
+        description: "Measures performance of CSS Transitions, with a transition on 20 properties but only 'left' and 'top' changing.",
+        done: onCompletedRun,
+        unit: 'fps'
+    });
 
     // Create the particles
     for (var i = 0; i < maxParticles; i++) {