Keep a separate list of :host(-context) rules.

Source/core/css/ElementRuleCollector.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.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 110 matching lines @@
     void setMatchingUARules(bool matchingUARules) { m_matchingUARules = matchingUARules; }
     void setScopeContainsLastMatchedElement(bool scopeContainsLastMatchedElement) { m_scopeContainsLastMatchedElement = scopeContainsLastMatchedElement; }
     bool scopeContainsLastMatchedElement() const { return m_scopeContainsLastMatchedElement; }
     bool hasAnyMatchingRules(RuleSet*);
 
     MatchResult& matchedResult();
     PassRefPtrWillBeRawPtr<StyleRuleList> matchedStyleRuleList();
     PassRefPtrWillBeRawPtr<CSSRuleList> matchedCSSRuleList();
 
     void collectMatchingRules(const MatchRequest&, RuleRange&, CascadeScope = ignoreCascadeScope, CascadeOrder = ignoreCascadeOrder, bool matchingTreeBoundaryRules = false);
+    void collectMatchingShadowHostRules(const MatchRequest&, RuleRange&, CascadeScope = ignoreCascadeScope, CascadeOrder = ignoreCascadeOrder, bool matchingTreeBoundaryRules = false);
     void sortAndTransferMatchedRules();
     void clearMatchedRules();
     void addElementStyleProperties(const StylePropertySet*, bool isCacheable = true);
 
 private:
     void collectRuleIfMatches(const RuleData&, CascadeScope, CascadeOrder, const MatchRequest&, RuleRange&);
 
     template<typename RuleDataListType>
     void collectMatchingRulesForList(const RuleDataListType* rules, CascadeScope cascadeScope, CascadeOrder cascadeOrder, const MatchRequest& matchRequest, RuleRange& ruleRange)
     {
@@ skipping 33 matching lines @@
 
     // Output.
     RefPtrWillBeMember<StaticCSSRuleList> m_cssRuleList;
     RefPtrWillBeMember<StyleRuleList> m_styleRuleList;
     MatchResult m_result;
 };
 
 } // namespace blink
 
 #endif // ElementRuleCollector_h