sheriff: mark callback-iframe.html as timeout on mac

Bug: 1156175
Change-Id: I948d76797152ab038beefc6c0cf85621037f593d
Tbr: avi@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575229Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834314}

third_party/blink/web_tests/TestExpectations

@@ -5889,3 +5889,6 @@ crbug.com/1155771 [ Linux ] external/wpt/dom/events/AddEventListenerOptions-sign
 
 # DevTools roll
 crbug.com/1144171 http/tests/devtools/report-API-errors.js [ Skip ]
+
+# Sheriff 2020-12-07
+crbug.com/1156175 external/wpt/requestidlecallback/callback-iframe.html [ Pass Timeout ]