Findit identified the culprit r561872 with confidence 100.0% in the config "chromium.mac / Mac10.11 Tests"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy1QELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKeAWNocm9taXVtLm1hYy9NYWMxMC4xMSBUZXN0cy8yNjYyMi9zeW5jX2ludGVncmF0aW9uX3Rlc3RzIG9uIE1hYy0xMC4xMS9WSGR2UTJ4cFpXNTBVSEpsWm1WeVpXNWpaWE5UZVc1alZHVnpkQzVNWVhSbFVtVm5hWE4wWlhKbFpGQnlaV1p6VTJodmRXeGtVM2x1WTE5Rk1rVlVaWE4wDAsSE01hc3RlckZsYWtlQW5hbHlzaXMYAQw


Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).
Feedback is welcome! Please use component Tools>Test>FindIt>Flakiness

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/86ec5da0933a8ac2ded15071986c40b5cbf76112

commit 86ec5da0933a8ac2ded15071986c40b5cbf76112
Author: Findit <findit-for-me@appspot.gserviceaccount.com>
Date: Fri May 25 17:04:21 2018

Revert "Support late registration of syncable preferences."

This reverts commit 80bae8d5eb3b978b48de9aaf635926a90357633e.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 561872 as the
culprit for flakes in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/flake/flake-culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyQwsSDEZsYWtlQ3VscHJpdCIxY2hyb21pdW0vODBiYWU4ZDVlYjNiOTc4YjQ4ZGU5YWFmNjM1OTI2YTkwMzU3NjMzZQw

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.mac/Mac10.11%20Tests/26622

Sample Failed Step: sync_integration_tests on Mac-10.11

Sample Flaky Test: TwoClientPreferencesSyncTest.LateRegisteredPrefsShouldSync_E2ETest

Original change's description:
> Support late registration of syncable preferences.
> 
> Today, there's already a race when registering a preference
> through the mojo preference service: the asynchronous
> registration call might be slower than starting sync. As
> more services are being implemented, those scenarios are
> getting more likely -- especially as services could be
> started at arbitrary times.
> 
> Before this path, the synced preferences implementation did
> not support this scenario. This patch changes the synced
> preference implementation to receive and persist remote
> data for preferences before they get registered. Currently,
> this only applies for a whitelisted set of preferences.
> More details about the approach can be found in the design
> doc linked from the crbug/840332.
> 
> Bug: 840332
> Change-Id: I90f046d7b59098a303e7b92350cc29e5de9cc244
> Reviewed-on: https://chromium-review.googlesource.com/1064750
> Commit-Queue: Tim Schumann <tschumann@chromium.org>
> Reviewed-by: Dominic Battré <battre@chromium.org>
> Reviewed-by: Ilya Sherman <isherman@chromium.org>
> Reviewed-by: Marc Treib <treib@chromium.org>
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#561872}

Change-Id: I50c2cc6742cf32ea96af4c30e9d857e6dac56144
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 840332,  846791 
Reviewed-on: https://chromium-review.googlesource.com/1073104
Cr-Commit-Position: refs/heads/master@{#561919}
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/chrome/browser/sync/test/integration/preferences_helper.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/chrome/browser/sync/test/integration/preferences_helper.h
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/chrome/browser/sync/test/integration/single_client_preferences_sync_test.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/chrome/browser/sync/test/integration/sync_test.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/chrome/browser/sync/test/integration/sync_test.h
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/chrome/browser/sync/test/integration/two_client_preferences_sync_test.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/pref_registry/pref_registry_syncable.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/pref_registry/pref_registry_syncable.h
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/prefs/pref_service.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/prefs/pref_service.h
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/sync_preferences/BUILD.gn
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/sync_preferences/pref_model_associator.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/sync_preferences/pref_model_associator.h
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/sync_preferences/pref_model_associator_unittest.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/sync_preferences/pref_service_syncable.cc
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/sync_preferences/pref_service_syncable.h
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/components/sync_preferences/pref_service_syncable_unittest.cc
[delete] https://crrev.com/300862e2b8998e8a4b0d2fa7c362f5c6b7ccdfb3/components/sync_preferences/unknown_user_pref_accessor.cc
[delete] https://crrev.com/300862e2b8998e8a4b0d2fa7c362f5c6b7ccdfb3/components/sync_preferences/unknown_user_pref_accessor.h
[modify] https://crrev.com/86ec5da0933a8ac2ded15071986c40b5cbf76112/tools/metrics/histograms/histograms.xml

Seems legit. FindIt gets a gold star.

Fixed with https://chromium-review.googlesource.com/1074688