[LayoutNG] fast/scrolling/unscrollable-layer-subpixel-size-with-negative-overflow.html is also flaky.

The virtual/fractional_scrolling_threaded/ version was already marked as
such, but the main test has similar issues. Pass rate is about 80%
(tested locally). The width of the yellow box is sometimes off by one
pixel.

TBR=kojii@chromium.org

Change-Id: Id7d7fd54ed1854557e596a3475885dc51ea2dae4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1605992
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658711}
1 file changed