StyleResolver should update RuleSets lazily.

Source/core/dom/Document.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, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 1729 matching lines @@
         clearChildNeedsStyleRecalc();
         unscheduleStyleRecalc();
 
         // FIXME: SVG <use> element can schedule a recalc in the middle of an already running one.
         // See StyleEngine::updateActiveStyleSheets.
         if (m_styleEngine->needsUpdateActiveStylesheetsOnStyleRecalc())
             setNeedsStyleRecalc();
 
         if (m_styleEngine->hasResolver()) {
             // Pseudo element removal and similar may only work with these flags still set. Reset them after the style recalc.
-            m_styleEngine->resetCSSFeatureFlags(m_styleEngine->resolver()->ruleFeatureSet());
+            m_styleEngine->resetCSSFeatureFlags(m_styleEngine->resolver()->ensureRuleFeatureSet());
             m_styleEngine->resolver()->clearStyleSharingList();
         }
     }
 
     InspectorInstrumentation::didRecalculateStyle(cookie);
 
     // As a result of the style recalculation, the currently hovered element might have been
     // detached (for example, by setting display:none in the :hover style), schedule another mouseMove event
     // to check if any other elements ended up under the mouse pointer due to re-layout.
     if (hoverNode() && !hoverNode()->renderer() && frame())
@@ skipping 3448 matching lines @@
 void Document::modifiedStyleSheet(StyleSheet* sheet, RecalcStyleTime when, StyleResolverUpdateMode updateMode)
 {
     if (!isActive())
         return;
 
     styleEngine()->modifiedStyleSheet(sheet);
     styleResolverChanged(when, updateMode);
 }
 
 } // namespace WebCore