sheriff: Mark COOP wpt test as expected to fail on Mac

Marking
external/wpt/html/cross-origin-opener-policy/reporting/access-reporting/opener-accessed_openee-coop-ro.https.html
as expected to fail - see ${BUG}.

Following
https://chromium.googlesource.com/chromium/src/+/master/docs/testing/web_platform_tests.md#waterfall-failures-caused-by-automatic-imports

Bug: 1108728
TBR=clamy@chromium.org

Change-Id: If1921bb903b990e4185ddaf45f5aefadc6802128
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315679Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791187}

third_party/blink/web_tests/TestExpectations

@@ -6852,3 +6852,6 @@ crbug.com/1105172 http/tests/devtools/sources/debugger-breakpoints/use-possible-
 crbug.com/1107722 [ Mac ] http/tests/devtools/tracing/timeline-js/timeline-script-id.js [ Pass Failure ]
 crbug.com/1107722 [ Mac ] virtual/threaded/http/tests/devtools/tracing/timeline-misc/timeline-event-causes.js [ Pass Failure ]
 crbug.com/1107923 [ Mac10.15 ] inspector-protocol/debugger/wasm-streaming-url.js [ Pass Timeout Failure ]
+
+# Sheriff 2020-07-23
+crbug.com/1108728 [ Mac ] external/wpt/html/cross-origin-opener-policy/reporting/access-reporting/opener-accessed_openee-coop-ro.https.html [ Failure ]