sheriff: Mark http/tests/devtools/tracing/timeline-js/timeline-gc-event.js as flaky on Mac

http/tests/devtools/tracing/timeline-js/timeline-gc-event.js is flaky on
Mac builders, mark it as such.

Bug: 1108786
TBR=ulan@chromium.org

Change-Id: I40861cea4daee5d2dcf3fa4aa91c30beceef86b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315689Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791266}

third_party/blink/web_tests/TestExpectations

@@ -6870,3 +6870,4 @@ crbug.com/1046784 [ Linux ] http/tests/devtools/tracing/timeline-misc/timeline-e
 
 # 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 ]
+crbug.com/1108786 [ Mac ] http/tests/devtools/tracing/timeline-js/timeline-gc-event.js [ Pass Failure ]