StyleResolver should update RuleSets lazily.

Source/core/dom/ShadowTreeStyleSheetCollection.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 90 matching lines @@
     analyzeStyleSheetChange(updateMode, activeAuthorStyleSheets(), activeCSSStyleSheets, change);
 
     if (StyleResolver* styleResolver = document()->styleResolverIfExists()) {
         // FIXME: We might have already had styles in child treescope. In this case, we cannot use buildScopedStyleTreeInDocumentOrder.
         // Need to change "false" to some valid condition.
         styleResolver->setBuildScopedStyleTreeInDocumentOrder(false);
         if (change.styleResolverUpdateType != Additive) {
             // We should not destroy StyleResolver when we find any stylesheet update in a shadow tree.
             // In this case, we will reset rulesets created from style elements in the shadow tree.
             resetAllRuleSetsInTreeScope(styleResolver);
-            styleResolver->appendAuthorStyleSheets(0, activeCSSStyleSheets);
+            styleResolver->removePendingAuthorStyleSheets(m_activeAuthorStyleSheets);
+            styleResolver->lazyAppendAuthorStyleSheets(0, activeCSSStyleSheets);
         } else {
-            styleResolver->appendAuthorStyleSheets(m_activeAuthorStyleSheets.size(), activeCSSStyleSheets);
+            styleResolver->lazyAppendAuthorStyleSheets(m_activeAuthorStyleSheets.size(), activeCSSStyleSheets);
         }
     }
     if (change.requiresFullStyleRecalc)
         toShadowRoot(m_treeScope.rootNode())->host()->setNeedsStyleRecalc();
 
     m_scopingNodesForStyleScoped.didRemoveScopingNodes();
     m_activeAuthorStyleSheets.swap(activeCSSStyleSheets);
     m_styleSheetsForStyleSheetList.swap(styleSheets);
     updateUsesRemUnits();
 
     return change.requiresFullStyleRecalc;
 }
 
 }