Update layout documentation re: coordinate systems and writing mode.

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 110 matching lines @@
 //           ------------------------------------- v
 //
 //                 'bottom' / 'end' side
 //
 // See https://drafts.csswg.org/css-writing-modes-3/#text-flow for some
 // extra details.
 //
 // - physical coordinates with flipped block-flow direction: those are physical
 //   coordinates but we flipped the block direction. See
 //   LayoutBox::noOverflowRect.
-//   TODO(jchaffraix): I don't fully understand why we need this coordinate
-//   system someone should fill in those details.
+//
+// For more, see Source/core/layout/README.md ### Coordinate Spaces.
 class CORE_EXPORT LayoutBoxModelObject : public LayoutObject {
  public:
   LayoutBoxModelObject(ContainerNode*);
   ~LayoutBoxModelObject() override;
 
   // This is the only way layers should ever be destroyed.
   void destroyLayer();
 
   LayoutSize relativePositionOffset() const;
   LayoutSize relativePositionLogicalOffset() const {
@@ skipping 363 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