Fix fundamental compositing issue for scrollers with overlay controls

Stop forcing scrollers with overlay controls (overlay scrollbars and
resizers) to be self-painting PaintLayers. Now overlay controls are
painted in three ways:
- (unchanged) If the overlay controls (of a non-stacking-context
  scroller) need reordering, i.e. need to be painted after some
  descendants in containing-block order but not in paint-order,
  the overlay controls are still painted in
  PaintLayerPainter::PaintChildren() of the containing stacking context.
- Otherwise:
  - (unchanged) if the scroller has a self-painting PaintLayer, the
    overlay controls are painted in
    PaintLayerPainter::PaintLayerContent() after all children.
  - (new) if the scroller doesn't have a self-painting PaintLayer,
    the overlay controls are painted in the foreground paint phase.

Bug: 370604
Change-Id: Ifc0437f794ad57e860c8d73c6aeae06a81f58601
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3375382


Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958033}