Surprisingly, the MSAN bot doesn't show the test failure in the most recent run:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac%20ASan%2064%20Tests%20%281%29/47397

Hmm, I'll wait for the next non-MSAN build to see whether this was a very unlikely flake before reverting https://crrev.com/c/1361863.

Meanwhile: alexclarke, do you know whether this would make sense to be related to your change?

Okay, the next two builds on the other Mac bots also failed:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests/7599
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.12%20Tests/17522

The flakiness dashboard isn't really a help here:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=components_unittests&tests=SyncSchedulerImplTest.ScheduleClearServerData_FailsRetriesSucceeds

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b1525775f8eb45cd60230ff8329e99167011d979

commit b1525775f8eb45cd60230ff8329e99167011d979
Author: Friedrich Horschig [CET] <fhorschig@chromium.org>
Date: Tue Dec 11 14:45:00 2018

Revert "[Reland #3] Use the SequenceManager in ScopedTaskEnvironment"

This reverts commit d1317fc30ce4e9ef08c5197c0f67767dfe585df4.

Reason for revert: SyncSchedulerImplTest is failing and this is the most likely culprit. More information in the linked bug.

Bug: 913946

Original change's description:
> [Reland #3] Use the SequenceManager in ScopedTaskEnvironment
> 
> A reland of https://crrev.com/c/1324391
> 
> This is necessary because we want content::TestBrowserThreadBundle to
> own a BrowserUIThreadScheduler, but that also owns a ScopedTaskEnvironment
> and you can't have two SequenceManagers on the same thread.
> 
> This patch allows ScopedTaskEnvironment to optionally work with an
> externally owned SequenceManager solving the problem.
> 
> This implements https://docs.google.com/document/d/1y08C6JQ9Yta3EQXzwIqqIIKHq9500WV6CWFZzZfDx7I/edit?usp=drivesdk,
> 
> We now have the ability to mock time on the UI and IO threads.
> 
> RE HttpServiceTest change HttpServiceTest.MultipleRequests was failing
> mysteriously on fuschia on the bots only (not locally). IO_MOCK_TIME seems
> to have fixed this.
> 
> 
> TBR=gab@chromium.org,fdoray@chromium.org,wez@chromium.org
> 
> Bug: 863341, 891670, 708584
> Change-Id: I95444a0a50f8b577c5fae62c12c8423c7e8f21d6
> Reviewed-on: https://chromium-review.googlesource.com/c/1361863
> Commit-Queue: Alex Clarke <alexclarke@chromium.org>
> Reviewed-by: Alex Clarke <alexclarke@chromium.org>
> Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#615496}

TBR=wez@chromium.org,gab@chromium.org,fdoray@chromium.org,skyostil@chromium.org,alexclarke@chromium.org

Change-Id: If5ff0362758b5270eb3113d2fd6eeeb7facc37c3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 863341, 891670, 708584
Reviewed-on: https://chromium-review.googlesource.com/c/1371811
Reviewed-by: Friedrich Horschig [CET] <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615520}
[modify] https://crrev.com/b1525775f8eb45cd60230ff8329e99167011d979/base/test/scoped_task_environment.cc
[modify] https://crrev.com/b1525775f8eb45cd60230ff8329e99167011d979/base/test/scoped_task_environment.h
[modify] https://crrev.com/b1525775f8eb45cd60230ff8329e99167011d979/base/test/scoped_task_environment_unittest.cc
[modify] https://crrev.com/b1525775f8eb45cd60230ff8329e99167011d979/webrunner/net_http/http_service_unittest.cc

Could well be related. Unfortunately the Mac & iOS message pumps don't behave the same way the other ones do which is causing us no end of trouble.

 Issue 913957  has been merged into this issue.

FYI: the MSAN builder hasn't shown another failure and Mac 10.13 also cycled green BEFORE the revert landed:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests/7600

If Mac 10.12 does the same, this could really have been an occurrence of three flakes with very extraordinary timing.

Mac 10.12 failed continuously until the revert was in. So the revert seemed to have effect and the original CL might just increase the flakiness a lot.

I did not notice any other test that broke by the original CL and cycled green with the revert. And neither did findit. Removing from sheriff queue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2ded172c0daa913d3d6e0b118d86ed38210cf06f

commit 2ded172c0daa913d3d6e0b118d86ed38210cf06f
Author: Alex Clarke <alexclarke@chromium.org>
Date: Thu Dec 13 12:55:20 2018

[Reland #4] Use the SequenceManager in ScopedTaskEnvironment

A reland of https://crrev.com/c/1324391

This is necessary because we want content::TestBrowserThreadBundle to
own a BrowserUIThreadScheduler, but that also owns a ScopedTaskEnvironment
and you can't have two SequenceManagers on the same thread.

This patch allows ScopedTaskEnvironment to optionally work with an
externally owned SequenceManager solving the problem.

This implements https://docs.google.com/document/d/1y08C6JQ9Yta3EQXzwIqqIIKHq9500WV6CWFZzZfDx7I/edit?usp=drivesdk,

We now have the ability to mock time on the UI and IO threads.

HttpServiceTest had been changed because HttpServiceTest.MultipleRequests was
failing mysteriously on fuchsia on the bots only (not locally). IO_MOCK_TIME
seems to have fixed this.

SyncSchedulerImplTest has been changed to use MOCK_TIME because it was
flaky on Mac. This change required base::TimeTicks::Now() to be overridden.
These tests should now run faster which I understand was a historical problem
(see  https://crbug.com/402212 ).

The chromeos test changes are needed because C++ requires a default constructor
to const construct an object.  ScopedTaskEnvironment has a template vararg
constructor which due to a defect in the C++ standard isn't recognized for this
purpose.  C++ will happily let you default construct a non-const object using
it however.

TBR=khorimoto@chromium.org, wez@chromium.org, jkrcal@chromium.org

Bug: 863341, 891670, 913946, 911928
Change-Id: Ib7e58223b531cf85aded11b75bd09c47a0993391
Reviewed-on: https://chromium-review.googlesource.com/c/1371899
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616280}
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/base/test/scoped_task_environment.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/base/test/scoped_task_environment.h
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/base/test/scoped_task_environment_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/components/nearby/count_down_latch_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/components/nearby/settable_future_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/components/tether/asynchronous_shutdown_object_container_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/components/tether/connection_preserver_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/components/tether/crash_recovery_manager_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/components/tether/host_scanner_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/components/tether/wifi_hotspot_disconnector_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/multidevice_setup/account_status_change_delegate_notifier_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/multidevice_setup/multidevice_setup_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/multidevice_setup/multidevice_setup_service_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/multidevice_setup/privileged_host_device_setter_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/active_connection_manager_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/authenticated_channel_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/ble_advertiser_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/ble_listener_operation_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/ble_synchronizer_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/client_connection_parameters_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/connect_to_device_operation_base_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/connection_attempt_base_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/multiplexed_channel_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/pending_ble_initiator_connection_request_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/pending_ble_listener_connection_request_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/pending_connection_manager_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/pending_connection_request_base_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/public/cpp/client/client_channel_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/public/cpp/client/secure_channel_client_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/secure_channel_service_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/chromeos/services/secure_channel/single_client_message_proxy_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/components/sync/engine_impl/sync_scheduler_impl_unittest.cc
[modify] https://crrev.com/2ded172c0daa913d3d6e0b118d86ed38210cf06f/webrunner/net_http/http_service_unittest.cc