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At least when using mock scrollbars from web tests (and maybe there are
real use cases too?), NeedsScrollbarReconstruction() will return true
after layout, which will make us remove the current scrollbars. Legacy
layout will re-lay out and eventually end up with updating the scrollbar
rectangles. NG cannot re-lay out from
PaintLayerScrollableArea::UpdateAfterLayout() like legacy does, but we
do need to update the scrollbar rectangles anyway. NG has its own
scrollbar size change detection mechanism, but in this particular case,
the scrollbars would have the exact same size with the default skin and
with the mock scrollbars, so no change would be detected, and there'd be
no re-layout, and no call to UpdateScrollDimensions().

Without this fix, scrollbars/rtl/overflow-scroll-rtl.html
would fail with
https://chromium-review.googlesource.com/c/chromium/src/+/1297367

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I15e159e0c26ab9fe8469ae8ad2c0af0dabe7417b
Reviewed-on: https://chromium-review.googlesource.com/c/1309741Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605116}