Fill in descriptions on relevant blink_perf tests

PerformanceTests/CSS/StyleSheetInsert-bootstrap.html

Index: PerformanceTests/CSS/StyleSheetInsert-bootstrap.html
diff --git a/PerformanceTests/CSS/StyleSheetInsert-bootstrap.html b/PerformanceTests/CSS/StyleSheetInsert-bootstrap.html
index 692ac643d5b11ccf9586d6342c4581d1d25a429c..1b0987cf9d4482b741fc7e53894c828b690f20f8 100644
--- a/PerformanceTests/CSS/StyleSheetInsert-bootstrap.html
+++ b/PerformanceTests/CSS/StyleSheetInsert-bootstrap.html
@@ -15,12 +15,15 @@ function loadText(path) {
 }
 var styleText = loadText("resources/bootstrap.min.css");
 
-PerfTestRunner.measureRunsPerSecond({run:function() {
-    var testDoc = document.getElementsByTagName("iframe")[0].contentDocument;
-    testDoc.documentElement.innerHTML = "";
-    var style = testDoc.createElement("style");
-    style.textContent = styleText;
-    testDoc.documentElement.appendChild(style);
-}});
+PerfTestRunner.measureRunsPerSecond({
+    description: "Measures performance of inserting a style elemenet ocntaining bootstrap's CSS into an iframe.",
+    run:function() {
+        var testDoc = document.getElementsByTagName("iframe")[0].contentDocument;
+        testDoc.documentElement.innerHTML = "";
+        var style = testDoc.createElement("style");
+        style.textContent = styleText;
+        testDoc.documentElement.appendChild(style);
+    }
+});
 </script>
 </html>