Avoid touching z-index in StyleAdjuster by using an isStackingContext flag instead

third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 665 matching lines @@
 {
     const LocalFrame* frame = document.frame();
 
     RefPtr<ComputedStyle> documentStyle = ComputedStyle::create();
     documentStyle->setRTLOrdering(document.visuallyOrdered() ? VisualOrder : LogicalOrder);
     documentStyle->setZoom(frame && !document.printing() ? frame->pageZoomFactor() : 1);
     FontDescription documentFontDescription = documentStyle->getFontDescription();
     documentFontDescription.setLocale(document.contentLanguage());
     documentStyle->setFontDescription(documentFontDescription);
     documentStyle->setZIndex(0);
+    documentStyle->setIsStackingContext(true);
     documentStyle->setUserModify(document.inDesignMode() ? READ_WRITE : READ_ONLY);
     // These are designed to match the user-agent stylesheet values for the document element
     // so that the common case doesn't need to create a new ComputedStyle in
     // Document::inheritHtmlAndBodyElementStyles.
     documentStyle->setDisplay(BLOCK);
     documentStyle->setPosition(AbsolutePosition);
 
     document.setupFontBuilder(*documentStyle);
 
     return documentStyle.release();
@@ skipping 1038 matching lines @@
     visitor->trace(m_siblingRuleSet);
     visitor->trace(m_uncommonAttributeRuleSet);
     visitor->trace(m_watchedSelectorsRules);
     visitor->trace(m_treeBoundaryCrossingScopes);
     visitor->trace(m_styleSharingLists);
     visitor->trace(m_pendingStyleSheets);
     visitor->trace(m_document);
 }
 
 } // namespace blink