Skip dom traversal for getComputedStyle with clean render tree.

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 1878 matching lines @@
     ASSERT(!childNeedsStyleRecalc());
     ASSERT(inStyleRecalc());
     m_lifecycle.advanceTo(DocumentLifecycle::StyleClean);
 }
 
 void Document::updateRenderTreeForNodeIfNeeded(Node* node)
 {
     ASSERT(node);
     if (!node->canParticipateInComposedTree())
         return;
+    if (!needsRenderTreeUpdate())
+        return;
 
     bool needsRecalc = needsFullRenderTreeUpdate() || node->needsStyleRecalc() || node->needsStyleInvalidation();
 
     if (!needsRecalc) {
         for (const ContainerNode* ancestor = NodeRenderingTraversal::parent(*node); ancestor && !needsRecalc; ancestor = NodeRenderingTraversal::parent(*ancestor))
             needsRecalc = ancestor->needsStyleRecalc() || ancestor->needsStyleInvalidation() || ancestor->needsAdjacentStyleRecalc();
     }
 
     if (needsRecalc)
         updateRenderTreeIfNeeded();
@@ skipping 3962 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().string().utf8().data());
 }
 #endif