message_center_unittests failing on chromium.mac/Mac10.12 Tests

Builders failed on: 
- Mac10.12 Tests: 
  https://build.chromium.org/p/chromium.mac/builders/Mac10.12%20Tests


This test appears to be flaky on the test bot. (Wild speculation --
 perhaps the timeouts are too close (1ms, 3ms, 5ms) that occasionally multiple timouts fire in a single iteration of the runloop?) I can't reproduce on my MacBook, but the test bot is on a VM, so timing variance may be much higher there.

Flakiness dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=message_center_unittests&tests=PopupTimersControllerStartMultipleTimers

Recent failures:
https://uberchromegw.corp.google.com/i/chromium.mac/builders/Mac10.12%20Tests/builds/341
https://uberchromegw.corp.google.com/i/chromium.mac/builders/Mac10.12%20Tests/builds/340
https://uberchromegw.corp.google.com/i/chromium.mac/builders/Mac10.12%20Tests/builds/325

Failure (from most recent build)
[ RUN      ] MessageCenterImplTest.PopupTimersControllerStartMultipleTimers
../../ui/message_center/message_center_impl_unittest.cc:326: Failure
Value of: "test2"
Expected: popup_timers_controller->last_id()
Which is: "test3"
[  FAILED  ] MessageCenterImplTest.PopupTimersControllerStartMultipleTimers (4 ms)

(Same failure on all three retries as well -- always sees "test3"; presumably test2 and test3 have both fired)