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

third_party/WebKit/Source/core/paint/PaintLayerStackingNode.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 41 matching lines @@
 #include <memory>
 
 namespace blink {
 
 class PaintLayer;
 class PaintLayerCompositor;
 class ComputedStyle;
 class LayoutBoxModelObject;
 
 // PaintLayerStackingNode represents a stacked element which is either a
-// stacking context (i.e. an element with non-auto z-index) or a positioned
-// element with auto z-index which is treated as a stacking context but
-// doesn't contain other stacked elements.
+// stacking context or a positioned element.
 // See https://chromium.googlesource.com/chromium/src.git/+/master/third_party/WebKit/Source/core/paint/README.md
 // for more details of stacked elements.
 //
 // Stacked elements are the basis for the CSS painting algorithm. The paint
 // order is determined by walking stacked elements in an order defined by
 // ‘z-index’. This walk is interleaved with non-stacked contents.
 // See CSS 2.1 appendix E for the actual algorithm
 // http://www.w3.org/TR/CSS21/zindex.html
 // See also PaintLayerPainter (in particular paintLayerContents) for
 // our implementation of the walk.
@@ skipping 156 matching lines @@
 
 private:
     PaintLayerStackingNode* m_stackingNode;
     bool m_previousMutationAllowedState;
 };
 #endif
 
 } // namespace blink
 
 #endif // PaintLayerStackingNode_h