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

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 45 matching lines @@
 
     const TreeScope& treeScope() const { return *m_scope; }
     ScopedStyleResolver* parent() const;
 
 public:
     const StyleRuleKeyframes* keyframeStylesForAnimation(const StringImpl* animationName);
     void addKeyframeStyle(PassRefPtrWillBeRawPtr<StyleRuleKeyframes>);
 
     unsigned appendCSSStyleSheet(CSSStyleSheet*);
     void collectMatchingAuthorRules(ElementRuleCollector&, bool includeEmptyRules, CascadeScope, CascadeOrder = ignoreCascadeOrder);
+    void collectMatchingShadowHostRules(ElementRuleCollector&, bool includeEmptyRules, CascadeScope, CascadeOrder = ignoreCascadeOrder);
     void matchPageRules(PageRuleCollector&);
     void collectFeaturesTo(RuleFeatureSet&, HashSet<const StyleSheetContents*>& visitedSharedStyleSheetContents) const;
     void resetAuthorStyle();
     void collectViewportRulesTo(StyleResolver*) const;
 
     void trace(Visitor*);
 
 private:
     explicit ScopedStyleResolver(TreeScope& scope)
         : m_scope(scope)
     {
     }
 
     RawPtrWillBeMember<TreeScope> m_scope;
 
     WillBeHeapVector<RawPtrWillBeMember<CSSStyleSheet> > m_authorStyleSheets;
 
     typedef WillBeHeapHashMap<const StringImpl*, RefPtrWillBeMember<StyleRuleKeyframes> > KeyframesRuleMap;
     KeyframesRuleMap m_keyframesRuleMap;
 };
 
 } // namespace blink
 
 #endif // ScopedStyleResolver_h