Only process each StyleSheetContents once when collecting features during style resolution

PerformanceTests/ShadowDOM/shadow-style-share-attr-selectors.html

Index: PerformanceTests/ShadowDOM/shadow-style-share-attr-selectors.html
diff --git a/PerformanceTests/ShadowDOM/shadow-style-share.html b/PerformanceTests/ShadowDOM/shadow-style-share-attr-selectors.html
similarity index 84%
copy from PerformanceTests/ShadowDOM/shadow-style-share.html
copy to PerformanceTests/ShadowDOM/shadow-style-share-attr-selectors.html
index e6eb37d7d18a57b477c3301296f9ddec9ec940f0..0aab52580a08fab6cc50170af830243e0ad26855 100644
--- a/PerformanceTests/ShadowDOM/shadow-style-share.html
+++ b/PerformanceTests/ShadowDOM/shadow-style-share-attr-selectors.html
@@ -6,7 +6,7 @@
 </style>
 <script src="../resources/runner.js"></script>
 <script>
-  var listSize = 100;
+  var listSize = 1000;
   window.onload = function() {
     PerfTestRunner.measureTime({
       run: function() {
@@ -30,17 +30,21 @@
   }
 </script>
 <template id="tmpl">
+  <!-- None of these styles match, but that's on purpose -->
   <style>
-    .foo .bar .foo.bar.baz {
+    [foo] {
       color: blue;
     }
-    .bar.baz {
+    [bar] {
       color: red;
     }
+    [baz] {
+      color: green;
+    }
   </style>
-  <div class="foo">
-    <div class="bar">
-      <div class="foo bar baz">item</div>
+  <div>
+    <div>
+      <div>item</div>
     </div>
   </div>
   <content></content>