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

Source/core/css/RuleSet.h

Index: Source/core/css/RuleSet.h
diff --git a/Source/core/css/RuleSet.h b/Source/core/css/RuleSet.h
index 99122d42f62a6f53f62c8ecf3c91af8b70175f86..e99b973c3c0c125f9b49e6ca4d95a6dfaf164a0c 100644
--- a/Source/core/css/RuleSet.h
+++ b/Source/core/css/RuleSet.h
@@ -144,6 +144,7 @@ public:
     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; }
@@ -218,6 +219,7 @@ private:
     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;