Implement style invalidation tree walk for targeted style recalc upon class change.

Source/core/css/resolver/StyleResolver.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 185 matching lines @@
     bool checkRegionStyle(Element* regionElement);
 
     // FIXME: Rename to reflect the purpose, like didChangeFontSize or something.
     void invalidateMatchedPropertiesCache();
 
     void notifyResizeForViewportUnits();
 
     // Exposed for RenderStyle::isStyleAvilable().
     static RenderStyle* styleNotYetAvailable() { return s_styleNotYetAvailable; }
 
-    const RuleFeatureSet& ensureRuleFeatureSet()
+    RuleFeatureSet& ensureRuleFeatureSet()
     {
         if (hasPendingAuthorStyleSheets())
             appendPendingAuthorStyleSheets();
         return m_features;
     }
 
     StyleSharingList& styleSharingList() { return m_styleSharingList; }
 
     bool hasRulesForId(const AtomicString&) const;
 
@@ skipping 126 matching lines @@
         SelectorChecker::SelectorCheckingContext selectorCheckingContext(s, regionElement, SelectorChecker::VisitedMatchDisabled);
         if (selectorChecker.match(selectorCheckingContext, DOMSiblingTraversalStrategy()) == SelectorChecker::SelectorMatches)
             return true;
     }
     return false;
 }
 
 } // namespace WebCore
 
 #endif // StyleResolver_h