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

Source/core/css/RuleSet.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 210 matching lines @@
             ASSERT_NOT_REACHED();
             return true;
         }
     }
 
     if (!tagName.isEmpty()) {
         addToRuleSet(tagName, ensurePendingRules()->tagRules, ruleData);
         return true;
     }
 
+    if (component.isHostPseudoClass()) {
+        m_shadowHostRules.append(ruleData);
+        return true;
+    }
     return false;
 }
 
 void RuleSet::addRule(StyleRule* rule, unsigned selectorIndex, AddRuleFlags addRuleFlags)
 {
     RuleData ruleData(rule, selectorIndex, m_ruleCount++, addRuleFlags);
     m_features.collectFeaturesFromRuleData(ruleData);
 
     if (!findBestRuleSetAndAdd(ruleData.selector(), ruleData)) {
         // If we didn't find a specialized map to stick it in, file under universal rules.
@@ skipping 106 matching lines @@
     ASSERT(m_pendingRules);
     OwnPtrWillBeRawPtr<PendingRuleMaps> pendingRules = m_pendingRules.release();
     compactPendingRules(pendingRules->idRules, m_idRules);
     compactPendingRules(pendingRules->classRules, m_classRules);
     compactPendingRules(pendingRules->tagRules, m_tagRules);
     compactPendingRules(pendingRules->shadowPseudoElementRules, m_shadowPseudoElementRules);
     m_linkPseudoClassRules.shrinkToFit();
     m_cuePseudoRules.shrinkToFit();
     m_focusPseudoClassRules.shrinkToFit();
     m_universalRules.shrinkToFit();
+    m_shadowHostRules.shrinkToFit();
     m_pageRules.shrinkToFit();
     m_viewportRules.shrinkToFit();
     m_fontFaceRules.shrinkToFit();
     m_keyframesRules.shrinkToFit();
     m_treeBoundaryCrossingRules.shrinkToFit();
     m_shadowDistributedRules.shrinkToFit();
 }
 
 void MinimalRuleData::trace(Visitor* visitor)
 {
@@ skipping 19 matching lines @@
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_idRules);
     visitor->trace(m_classRules);
     visitor->trace(m_tagRules);
     visitor->trace(m_shadowPseudoElementRules);
     visitor->trace(m_linkPseudoClassRules);
     visitor->trace(m_cuePseudoRules);
     visitor->trace(m_focusPseudoClassRules);
     visitor->trace(m_universalRules);
+    visitor->trace(m_shadowHostRules);
     visitor->trace(m_features);
     visitor->trace(m_pageRules);
     visitor->trace(m_viewportRules);
     visitor->trace(m_fontFaceRules);
     visitor->trace(m_keyframesRules);
     visitor->trace(m_treeBoundaryCrossingRules);
     visitor->trace(m_shadowDistributedRules);
     visitor->trace(m_viewportDependentMediaQueryResults);
     visitor->trace(m_pendingRules);
 #ifndef NDEBUG
     visitor->trace(m_allRules);
 #endif
 #endif
 }
 
 #ifndef NDEBUG
 void RuleSet::show()
 {
     for (const auto& rule: m_allRules)
         rule.selector().show();
 }
 #endif
 
 } // namespace blink