Only process each StyleSheetContents once when collecting features during style resolution

Source/core/css/resolver/ScopedStyleResolver.h

 /*
  * 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 13 matching lines @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef ScopedStyleResolver_h
 #define ScopedStyleResolver_h
 
 #include "core/css/ElementRuleCollector.h"
 #include "core/css/RuleSet.h"
 #include "core/dom/ContainerNode.h"
 #include "wtf/HashMap.h"
+#include "wtf/HashSet.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace WebCore {
 
 class MediaQueryEvaluator;
 class PageRuleCollector;
 class ShadowRoot;
 class StyleSheetContents;
 
@@ skipping 10 matching lines @@
     void setParent(ScopedStyleResolver* newParent) { m_parent = newParent; }
     ScopedStyleResolver* parent() { return m_parent; }
 
 public:
     const StyleRuleKeyframes* keyframeStylesForAnimation(const StringImpl* animationName);
     void addKeyframeStyle(PassRefPtrWillBeRawPtr<StyleRuleKeyframes>);
 
     void collectMatchingAuthorRules(ElementRuleCollector&, bool includeEmptyRules, bool applyAuthorStyles, CascadeScope, CascadeOrder = ignoreCascadeOrder);
     void matchPageRules(PageRuleCollector&);
     void addRulesFromSheet(CSSStyleSheet*, const MediaQueryEvaluator&, StyleResolver*);
-    void collectFeaturesTo(RuleFeatureSet&);
+    void collectFeaturesTo(RuleFeatureSet&, HashSet<const StyleSheetContents*>& visitedSharedStyleSheetContents);
     void resetAuthorStyle();
     void collectViewportRulesTo(StyleResolver*) const;
 
 private:
     explicit ScopedStyleResolver(ContainerNode& scopingNode) : m_scopingNode(scopingNode), m_parent(0) { }
 
     ContainerNode& m_scopingNode;
     ScopedStyleResolver* m_parent;
 
     Vector<CSSStyleSheet*> m_authorStyleSheets;
 
     typedef WillBePersistentHeapHashMap<const StringImpl*, RefPtrWillBeMember<StyleRuleKeyframes> > KeyframesRuleMap;
     KeyframesRuleMap m_keyframesRuleMap;
 };
 
 } // namespace WebCore
 
 #endif // ScopedStyleResolver_h