Mark http/tests/devtools/tracing/worker-events.js as flaky on Mac10.13

TBR=chenwilliam@chromium.org
NOTRY=true

Bug: 860101
Change-Id: Ia4c9179dc2c287123c6781c57d6db7c1b4da6393
Reviewed-on: https://chromium-review.googlesource.com/1124734Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572433}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2586,6 +2586,9 @@ crbug.com/548765 http/tests/devtools/console-fetch-logging.js [ Failure Pass ]
 crbug.com/v8/7486 http/tests/devtools/startup/console/console-format-startup-bigint.js [ Skip ]
 crbug.com/v8/7486 http/tests/devtools/console/console-format-bigint.js [ Skip ]
 
+crbug.com/860101 [ Mac10.13 Debug ] http/tests/devtools/tracing/worker-events.js [ Failure Pass ]
+crbug.com/860101 [ Mac10.13 Debug ] virtual/threaded/http/tests/devtools/tracing/worker-events.js [ Failure Pass ]
+
 crbug.com/564109 [ Win ]  http/tests/webfont/font-display-intervention.html [ Pass Failure Timeout ]
 
 crbug.com/399951 http/tests/mime/javascript-mimetype-usecounters.html [ Pass Failure ]