[sheriff] Mark cross-origin-objects.html Flaky

We have been seeing the not-site-per-process variant timing out on Mac10.13

The external/wpt variant was already known to exhibit this failure, so
I'm marking this variant flaky as well.

TBR=timothygu@chromium.org
TEST=virtual/not-site-per-process/external/wpt/html/browsers/origin/cross-origin-objects/cross-origin-objects.html
Bug:1027434

Change-Id: Iedddebf91837957db7d571b6fcd158c61d87b23a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227868Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774813}

third_party/blink/web_tests/TestExpectations

@@ -5908,3 +5908,4 @@ crbug.com/1011811 http/tests/devtools/persistence/automapping-sourcemap.js [ Pas
 crbug.com/1088441 [ Mac ] http/tests/loading/bad-server-subframe.html [ Pass Failure ]
 crbug.com/1090822 [ Mac10.10 ] virtual/cascade/fast/forms/month/month-appearance-basic.html [ Pass Failure ]
 crbug.com/1083293 [ Linux ] media/video-aspect-ratio.html [ Pass Failure ]
+crbug.com/1027434 [ Mac10.13 ] virtual/not-site-per-process/external/wpt/html/browsers/origin/cross-origin-objects/cross-origin-objects.html [ Pass Timeout ]