Don't recalculate style synchronously on hover/active.

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 5317 matching lines @@
         if (ancestorNode && nodesToAddToChain[i] == ancestorNode)
             sawCommonAncestor = true;
         if (allowActiveChanges)
             nodesToAddToChain[i]->setActive(true);
         if (!sawCommonAncestor) {
             nodesToAddToChain[i]->setHovered(true);
             if (event && (ancestorHasCapturingMouseenterListener || nodesToAddToChain[i]->hasEventListeners(EventTypeNames::mouseenter)))
                 nodesToAddToChain[i]->dispatchMouseEvent(*event, EventTypeNames::mouseenter, 0, oldHoverNode.get());
         }
     }
-
-    updateRenderTreeIfNeeded();
 }
 
 bool Document::haveStylesheetsLoaded() const
 {
     return m_styleEngine->haveStylesheetsLoaded();
 }
 
 Locale& Document::getCachedLocale(const AtomicString& locale)
 {
     AtomicString localeKey = locale;
@@ skipping 177 matching lines @@
         (*it)->invalidateCache(attrName);
 }
 
 void Document::trace(Visitor* visitor)
 {
     Supplementable<Document>::trace(visitor);
     ContainerNode::trace(visitor);
 }
 
 } // namespace WebCore