Currently we paint overlay scrollbars with two passes of painting:
- The first pass is in the background phase in which we paint normal
  scrollbars but for overlay scrollbars we set a flag of dirty overlay
  scrollbars;
- The second pass is after all paint phases, which paint the overlay
  scrollbars "on top of everything else".

I think the complex logic is to ensure that the overlay scrollbars
were painted "on top of everything else", but that is neither always
true nor necessary. The overlay scrollbars can still be covered by
other composited layers above the composited layer it resides. This
causes inconsistent behaviors about z-ordering of overlay scrollbars
in composited and non-composited layers.

More importantly, it also causes bugs:

- During a paint, when a layer with overlay scrollbars didn't need
  repaint, we issue cached subsequence but miss setting the dirty
  overlay scrollbars flag, causing missing overlay scrollbars for the
  layer in the second pass.

- The second pass is initiated by FramePainter and
  CompositeLayerMapping. For CompositedAfterPaint we depend on the
  former, but that will paint all overlay scrollbars on top of
  everything else in the frame even if the scrollbar is supposed to
  be covered by other things and should not visible and actionable.