Fill in descriptions on relevant blink_perf tests

PerformanceTests/Animation/balls-transition-40-properties.html

Index: PerformanceTests/Animation/balls-transition-40-properties.html
diff --git a/PerformanceTests/Animation/balls-transition-40-properties.html b/PerformanceTests/Animation/balls-transition-40-properties.html
index 904874511e48047a2753d9b19d7a3358e533e03b..da62d575ce0d471c8dfc6a06e419b738c1fdeb34 100644
--- a/PerformanceTests/Animation/balls-transition-40-properties.html
+++ b/PerformanceTests/Animation/balls-transition-40-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 40 properties but only 'left' and 'top' changing.",
+        done: onCompletedRun,
+        unit: 'fps'
+    });
 
     // Create the particles
     for (var i = 0; i < maxParticles; i++) {