Bumped into this while investigating  issue 861529 .

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f9dbd272ab74d0c5b0975830d0e1d3bdc8e94ad0

commit f9dbd272ab74d0c5b0975830d0e1d3bdc8e94ad0
Author: Florin Malita <fmalita@chromium.org>
Date: Mon Jul 09 23:41:31 2018

Fix rendering of double outlines

Outlines should always be rectangular, and the outline paint code forces
the outer/inner radii to zero.

But the shared paint code in BoxBorderPainter::DrawDoubleBorder does not
enforce zero-radii when computing the intermediate double edges.

Update DrawDoubleBorder() to avoid rounded intermediate edges when the
exterior edges are not rounded.

Bug:  861816 
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I81cfccfbb3f14568a0aeacb065fc447f583f28c8
Reviewed-on: https://chromium-review.googlesource.com/1129334
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573529}
[add] https://crrev.com/f9dbd272ab74d0c5b0975830d0e1d3bdc8e94ad0/third_party/WebKit/LayoutTests/fast/borders/outline-rounded-border-expected.png
[add] https://crrev.com/f9dbd272ab74d0c5b0975830d0e1d3bdc8e94ad0/third_party/WebKit/LayoutTests/fast/borders/outline-rounded-border.html
[add] https://crrev.com/f9dbd272ab74d0c5b0975830d0e1d3bdc8e94ad0/third_party/WebKit/LayoutTests/platform/mac/fast/borders/outline-rounded-border-expected.png
[modify] https://crrev.com/f9dbd272ab74d0c5b0975830d0e1d3bdc8e94ad0/third_party/blink/renderer/core/paint/box_border_painter.cc