Mark test as flaky in TestExpectation

This test has been flaky for a while. See the bug for details.

TBR=arthursonzogni@chromium.org

Bug: 1133342
Change-Id: I115c2bd2c84958e2e9f46bd9c0f378756af3c9eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436764Reviewed-by: Yi Gu <yigu@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811753}

third_party/blink/web_tests/TestExpectations

@@ -6732,3 +6732,6 @@ crbug.com/1057060 virtual/compositor-threaded-percent-based-scrolling/fast/scrol
 crbug.com/1057060 virtual/threaded-prefer-compositing/fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Pass Failure Timeout ]
 crbug.com/1057060 virtual/scroll-unification/fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Pass Failure Timeout ]
 crbug.com/1045599 fast/css-grid-layout/flex-content-sized-columns-resize.html [ Pass Failure Timeout ]
+
+# Sheriff 2020-09-29
+crbug.com/1133342 external/wpt/html/cross-origin-opener-policy/reporting/access-reporting/reporting-observer.html [ Pass Failure Timeout ]
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