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When the scrollbar is on left and the box is narrower than
the scrollbar, the scrollbar overflows to left.
LayoutBox::AddLayoutOverflow() expects the overflow adjusted
by the amount the scrollbar overflows.

This affects a jQuery plugin that detects RTL scroll type[1].

[1] https://github.com/othree/jquery.rtl-scroll-type

Bug: 635619
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I70e7b1ecc20857ff10687afd7d5e3b7d70afb2dc
Reviewed-on: https://chromium-review.googlesource.com/1215365Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590138}