Rework mapToVisualRectInAncestorSpace to handle flipped blocks correctly.

third_party/WebKit/Source/core/layout/README.md

 # `Source/core/layout`
 
 ## Overflow rects and scroll offsets
 
 PaintLayerScrollableArea uses a "scroll origin" to conceptually represent the distance between
 the top-left corner of the box'es content rect and the top-left corner of its overflow rect
 when the box is scrolled to the logical beginning of its content (.e.g. all the way to the left for
 LTR, all the way to the right for RTL).  For left-to-right and top-to-bottom flows, the scroll
 origin is zero, i.e., the top/left of the overflow rect is at the same position as the top/left of
 the box'es content rect when scrolled to the beginning of flow.  For right-to-left and bottom-to-top
@@ skipping 59 matching lines @@
                     |          |          |/|
 writing-mode:       |          |          |/|
   vertical-rl       |          |          |/|
                     |          |          |/|
                     |          |          |/|
                     |          |          |/|
                     |__________|__________|/|
                     
                           overflow rect
                     |<--------------------->|
+
+## Coordinate Spaces
+
+TODO(wkorman): Document writing mode, particularly flipped blocks.