Mark timeline-gc-event.js as flaky on Linux

Bug: 1108786
Change-Id: Ie498760184517698d3ef0346f996fa09a7c44960
No-Try: True
Tbr: mahmadi@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2321305Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792005}

third_party/blink/web_tests/TestExpectations

@@ -6856,6 +6856,7 @@ 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 ]
+crbug.com/1108786 [ Linux ] http/tests/devtools/tracing/timeline-js/timeline-gc-event.js [ Pass Failure ]
 
 # Sheriff 2020-07-27
 crbug.com/1107944 [ Mac ] http/tests/devtools/tracing/timeline-paint/timeline-paint.js [ Pass Failure ]