Only process each StyleSheetContents once when collecting features during style resolution

Source/core/css/resolver/ScopedStyleResolver.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 60 matching lines @@
 {
     m_authorStyleSheets.append(cssSheet);
     StyleSheetContents* sheet = cssSheet->contents();
 
     AddRuleFlags addRuleFlags = resolver->document().securityOrigin()->canRequest(sheet->baseURL()) ? RuleHasDocumentSecurityOrigin : RuleHasNoSpecialState;
     const RuleSet& ruleSet = sheet->ensureRuleSet(medium, addRuleFlags);
     resolver->addMediaQueryResults(ruleSet.viewportDependentMediaQueryResults());
     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()
 {
     m_authorStyleSheets.clear();
     m_keyframesRuleMap.clear();
 }
 
 const StyleRuleKeyframes* ScopedStyleResolver::keyframeStylesForAnimation(const StringImpl* animationName)
 {
@@ skipping 51 matching lines @@
 
 void ScopedStyleResolver::collectViewportRulesTo(StyleResolver* resolver) const
 {
     if (!m_scopingNode.isDocumentNode())
         return;
     for (size_t i = 0; i < m_authorStyleSheets.size(); ++i)
         resolver->viewportStyleResolver()->collectViewportRules(&m_authorStyleSheets[i]->contents()->ruleSet(), ViewportStyleResolver::AuthorOrigin);
 }
 
 } // namespace WebCore