getComputedStyle should not always force a layout inside shadow trees.

third_party/WebKit/Source/core/css/CSSComputedStyleDeclaration.cpp

 /*
  * Copyright (C) 2004 Zack Rusin <zack@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2011 Sencha, Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ skipping 383 matching lines @@
 
   // The style recalc could have caused the styled node to be discarded or
   // replaced if it was a PseudoElement so we need to update it.
   styledNode = this->styledNode();
   LayoutObject* layoutObject = styledNode->layoutObject();
 
   const ComputedStyle* style = computeComputedStyle();
 
   bool forceFullLayout =
       isLayoutDependent(propertyID, style, layoutObject) ||
-      styledNode->isInShadowTree() ||
       (document.localOwner() &&
        document.styleEngine().hasViewportDependentMediaQueries());
 
   if (forceFullLayout) {
     document.updateStyleAndLayoutIgnorePendingStylesheetsForNode(styledNode);
     styledNode = this->styledNode();
     style = computeComputedStyle();
     layoutObject = styledNode->layoutObject();
   }
 
@@ skipping 147 matching lines @@
       "These styles are computed, and therefore the '" +
           getPropertyNameString(id) + "' property is read-only.");
 }
 
 DEFINE_TRACE(CSSComputedStyleDeclaration) {
   visitor->trace(m_node);
   CSSStyleDeclaration::trace(visitor);
 }
 
 }  // namespace blink