Only process each StyleSheetContents once when collecting features during style resolution

Source/core/css/resolver/ScopedStyleResolver.cpp

Index: Source/core/css/resolver/ScopedStyleResolver.cpp
diff --git a/Source/core/css/resolver/ScopedStyleResolver.cpp b/Source/core/css/resolver/ScopedStyleResolver.cpp
index 5763d0d923e72ee9ec79b7b92f5b30ebbf234451..e9693d0db983be5de1d7ef746151382d52539841 100644
--- a/Source/core/css/resolver/ScopedStyleResolver.cpp
+++ b/Source/core/css/resolver/ScopedStyleResolver.cpp
@@ -78,10 +78,13 @@ void ScopedStyleResolver::addRulesFromSheet(CSSStyleSheet* cssSheet, const Media
     resolver->processScopedRules(ruleSet, sheet->baseURL(), &m_scopingNode);
 }
 
-void ScopedStyleResolver::collectFeaturesTo(RuleFeatureSet& features)
+void ScopedStyleResolver::collectFeaturesTo(RuleFeatureSet& features, HashSet<const StyleSheetContents*>& visitedSharedStyleSheetContents)
 {
-    for (size_t i = 0; i < m_authorStyleSheets.size(); ++i)
-        features.add(m_authorStyleSheets[i]->contents()->ruleSet().features());
+    for (size_t i = 0; i < m_authorStyleSheets.size(); ++i) {
+        StyleSheetContents* contents = m_authorStyleSheets[i]->contents();
+        if (contents->hasOneClient() || visitedSharedStyleSheetContents.add(contents).isNewEntry)
+            features.add(contents->ruleSet().features());
+    }
 }
 
 void ScopedStyleResolver::resetAuthorStyle()