Fill in descriptions on relevant blink_perf tests

PerformanceTests/Parser/html-parser-threaded.html

Index: PerformanceTests/Parser/html-parser-threaded.html
diff --git a/PerformanceTests/Parser/html-parser-threaded.html b/PerformanceTests/Parser/html-parser-threaded.html
index c77444924813df4551c50f30d474387a0894c146..e6f2fc575378c4ec37542a9966278e5a63db8c10 100644
--- a/PerformanceTests/Parser/html-parser-threaded.html
+++ b/PerformanceTests/Parser/html-parser-threaded.html
@@ -13,7 +13,11 @@ iframe.style.display = "none";  // Prevent creation of the rendering tree, so we
 iframe.sandbox = '';  // Prevent external script loads which could cause write() to return before completing the parse.
 document.body.appendChild(iframe);
 
-PerfTestRunner.prepareToMeasureValuesAsync({done: onCompletedRun, unit: 'ms'});
+PerfTestRunner.prepareToMeasureValuesAsync({
+    description: "Measures performance of the threaded HTML parser (if available).",
+    done: onCompletedRun,
+    unit: 'ms'
+});
 
 iframe.onload = function() {
     var now = PerfTestRunner.now();