Update layout documentation re: coordinate systems and writing mode.

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

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
index 41271a7d2e677f8bf575b73165ebc9cea584050c..6e9dfa34126d79fe58e75c81f8c448e325e4640e 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
@@ -128,8 +128,8 @@ struct LayoutBoxModelObjectRareData {
 // - 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*);