Fill in descriptions on relevant blink_perf tests

PerformanceTests/ShadowDOM/shadow-style-share-with-distribution.html

Index: PerformanceTests/ShadowDOM/shadow-style-share-with-distribution.html
diff --git a/PerformanceTests/ShadowDOM/shadow-style-share-with-distribution.html b/PerformanceTests/ShadowDOM/shadow-style-share-with-distribution.html
index 8915fa6be5618d7263b01df9002903f99b65b671..3da6e0d50c109418ef552e5429849c69e6ad024e 100644
--- a/PerformanceTests/ShadowDOM/shadow-style-share-with-distribution.html
+++ b/PerformanceTests/ShadowDOM/shadow-style-share-with-distribution.html
@@ -16,6 +16,7 @@
     root.appendChild(document.createElement('content'));
 
     PerfTestRunner.measureTime({
+      description: "Measures performance of creating and rendering elements with shadow roots (contains class descendant selector styles).",
       setup: function() {
       },
       run: function() {
@@ -42,4 +43,4 @@
     <div class="bar baz">item</div>
   </div>
 </template>
-<section id="list"></section>
+<section id="list"></section>