Record box shadow as main thread scrolling reasons

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 179 matching lines @@
 
   virtual LayoutRect visualOverflowRect() const = 0;
 
   // Checks if this box, or any of it's descendants, or any of it's
   // continuations, will take up space in the layout of the page.
   bool hasNonEmptyLayoutSize() const;
   bool usesCompositedScrolling() const;
 
   // Returns which layers backgrounds should be painted into for overflow
   // scrolling boxes.
-  BackgroundPaintLocation backgroundPaintLocation() const;
+  BackgroundPaintLocation backgroundPaintLocation(uint32_t* reasons = 0) const;

[bokan, 2017/01/23 19:55:51]

nit: s/0/nullptr

Also, please document what the method puts in `reasons` in the comment.

[yigu, 2017/01/23 20:21:23]

Done.

 
   // These return the CSS computed padding values.
   LayoutUnit computedCSSPaddingTop() const {
     return computedCSSPadding(style()->paddingTop());
   }
   LayoutUnit computedCSSPaddingBottom() const {
     return computedCSSPadding(style()->paddingBottom());
   }
   LayoutUnit computedCSSPaddingLeft() const {
     return computedCSSPadding(style()->paddingLeft());
@@ skipping 315 matching lines @@
   std::unique_ptr<PaintLayer> m_layer;
 
   std::unique_ptr<LayoutBoxModelObjectRareData> m_rareData;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBoxModelObject, isBoxModelObject());
 
 }  // namespace blink
 
 #endif  // LayoutBoxModelObject_h