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

Source/core/css/RuleSet.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 126 matching lines @@
     const RuleFeatureSet& features() const { return m_features; }
 
     const WillBeHeapTerminatedArray<RuleData>* idRules(const AtomicString& key) const { ASSERT(!m_pendingRules); return m_idRules.get(key); }
     const WillBeHeapTerminatedArray<RuleData>* classRules(const AtomicString& key) const { ASSERT(!m_pendingRules); return m_classRules.get(key); }
     const WillBeHeapTerminatedArray<RuleData>* tagRules(const AtomicString& key) const { ASSERT(!m_pendingRules); return m_tagRules.get(key); }
     const WillBeHeapTerminatedArray<RuleData>* shadowPseudoElementRules(const AtomicString& key) const { ASSERT(!m_pendingRules); return m_shadowPseudoElementRules.get(key); }
     const WillBeHeapVector<RuleData>* linkPseudoClassRules() const { ASSERT(!m_pendingRules); return &m_linkPseudoClassRules; }
     const WillBeHeapVector<RuleData>* cuePseudoRules() const { ASSERT(!m_pendingRules); return &m_cuePseudoRules; }
     const WillBeHeapVector<RuleData>* focusPseudoClassRules() const { ASSERT(!m_pendingRules); return &m_focusPseudoClassRules; }
     const WillBeHeapVector<RuleData>* universalRules() const { ASSERT(!m_pendingRules); return &m_universalRules; }
+    const WillBeHeapVector<RuleData>* shadowHostRules() const { ASSERT(!m_pendingRules); return &m_shadowHostRules; }
     const WillBeHeapVector<RawPtrWillBeMember<StyleRulePage> >& pageRules() const { ASSERT(!m_pendingRules); return m_pageRules; }
     const WillBeHeapVector<RawPtrWillBeMember<StyleRuleViewport> >& viewportRules() const { ASSERT(!m_pendingRules); return m_viewportRules; }
     const WillBeHeapVector<RawPtrWillBeMember<StyleRuleFontFace> >& fontFaceRules() const { return m_fontFaceRules; }
     const WillBeHeapVector<RawPtrWillBeMember<StyleRuleKeyframes> >& keyframesRules() const { return m_keyframesRules; }
     const WillBeHeapVector<MinimalRuleData>& treeBoundaryCrossingRules() const { return m_treeBoundaryCrossingRules; }
     const WillBeHeapVector<MinimalRuleData>& shadowDistributedRules() const { return m_shadowDistributedRules; }
     const MediaQueryResultList& viewportDependentMediaQueryResults() const { return m_viewportDependentMediaQueryResults; }
 
     unsigned ruleCount() const { return m_ruleCount; }
 
@@ skipping 54 matching lines @@
     }
 
     CompactRuleMap m_idRules;
     CompactRuleMap m_classRules;
     CompactRuleMap m_tagRules;
     CompactRuleMap m_shadowPseudoElementRules;
     WillBeHeapVector<RuleData> m_linkPseudoClassRules;
     WillBeHeapVector<RuleData> m_cuePseudoRules;
     WillBeHeapVector<RuleData> m_focusPseudoClassRules;
     WillBeHeapVector<RuleData> m_universalRules;
+    WillBeHeapVector<RuleData> m_shadowHostRules;
     RuleFeatureSet m_features;
     WillBeHeapVector<RawPtrWillBeMember<StyleRulePage> > m_pageRules;
     WillBeHeapVector<RawPtrWillBeMember<StyleRuleViewport> > m_viewportRules;
     WillBeHeapVector<RawPtrWillBeMember<StyleRuleFontFace> > m_fontFaceRules;
     WillBeHeapVector<RawPtrWillBeMember<StyleRuleKeyframes> > m_keyframesRules;
     WillBeHeapVector<MinimalRuleData> m_treeBoundaryCrossingRules;
     WillBeHeapVector<MinimalRuleData> m_shadowDistributedRules;
 
     MediaQueryResultList m_viewportDependentMediaQueryResults;
 
     unsigned m_ruleCount;
     OwnPtrWillBeMember<PendingRuleMaps> m_pendingRules;
 
 #ifndef NDEBUG
     WillBeHeapVector<RuleData> m_allRules;
 #endif
 };
 
 } // namespace blink
 
 WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS(blink::RuleData);
 WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS(blink::MinimalRuleData);
 
 #endif // RuleSet_h