Implement proposed shadow tree cascade order.

Source/core/css/resolver/ScopedStyleResolver.cpp

 /*
  * 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 151 matching lines @@
     for (size_t i = 0; i < m_authorStyleSheets.size(); ++i) {
         ASSERT(m_authorStyleSheets[i]->ownerNode());
         MatchRequest matchRequest(&m_authorStyleSheets[i]->contents()->ruleSet(), includeEmptyRules, &m_scope->rootNode(), m_authorStyleSheets[i], i);
         collector.collectMatchingShadowHostRules(matchRequest, cascadeOrder);
     }
     collector.setScopeContainsLastMatchedElement(false);
 }
 
 void ScopedStyleResolver::collectMatchingTreeBoundaryCrossingRules(ElementRuleCollector& collector, bool includeEmptyRules, CascadeOrder cascadeOrder)
 {
+    if (!m_treeBoundaryCrossingRuleSet)
+        return;
+
+    ASSERT(!collector.scopeContainsLastMatchedElement());
+    collector.setScopeContainsLastMatchedElement(true);
+
     for (const auto& rules : *m_treeBoundaryCrossingRuleSet) {
         MatchRequest request(rules->m_ruleSet.get(), includeEmptyRules, &treeScope().rootNode(), rules->m_parentStyleSheet, rules->m_parentIndex);
         collector.collectMatchingRules(request, cascadeOrder, true);
     }
+
+    collector.setScopeContainsLastMatchedElement(false);
 }
 
 void ScopedStyleResolver::matchPageRules(PageRuleCollector& collector)
 {
     // Only consider the global author RuleSet for @page rules, as per the HTML5 spec.
     ASSERT(m_scope->rootNode().isDocumentNode());
     for (size_t i = 0; i < m_authorStyleSheets.size(); ++i)
         collector.matchPageRules(&m_authorStyleSheets[i]->contents()->ruleSet());
 }
 
@@ skipping 43 matching lines @@
     m_treeBoundaryCrossingRuleSet->append(RuleSubSet::create(parentStyleSheet, sheetIndex, ruleSetForScope.release()));
 }
 
 DEFINE_TRACE(ScopedStyleResolver::RuleSubSet)
 {
     visitor->trace(m_parentStyleSheet);
     visitor->trace(m_ruleSet);
 }
 
 } // namespace blink