Mark http/tests/inspector-protocol/page/page-events-associated.js test flaky

TBR=paulirish@chromium.org

Bug: 1163793
Change-Id: Ib5de555db523a6aaf48d4829b3323b8e3c8e22cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623731
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842391}

third_party/blink/web_tests/TestExpectations

@@ -5863,3 +5863,5 @@ crbug.com/1046784 http/tests/devtools/tracing/timeline-receive-response-event.js
 crbug.com/1164459 [ Mac ] virtual/synchronous_html_parser/external/wpt/preload/download-resources.html [ Pass Timeout ]
 crbug.com/1164568 [ Mac ] external/wpt/html/cross-origin-opener-policy/reporting/navigation-reporting/reporting-popup-same-origin-allow-popups-report-to.https.html [ Pass Failure ]
 
+# Sheriff 2021-01-12
+crbug.com/1163793 http/tests/inspector-protocol/page/page-events-associated.js [ Pass Failure ]