Combine ScrollbarPaintOffset into PaintOffsetTranslation

Now we create PaintOffsetTranslation for all scrollable boxes, so in
most cases ScrollbarPaintOffset is zero. This CL just extends
PaintOffsetTranslation to boxes having scrollbars but not scrollable
(e.g. overflow: scroll without overflow). This saves one node for
scrollable boxes and simplifies scrollbar painting.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ic62984310cfbcaa6f6a9de080bbc50c31f9c0433
Reviewed-on: https://chromium-review.googlesource.com/780401


Reviewed-by: Tien-Ren Chen <trchen@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518278}