Findit identified the culprit r569640 with confidence 70.0% in the config "chromium.win / Win10 Tests x64"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVypAELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCJuY2hyb21pdW0ud2luL1dpbjEwIFRlc3RzIHg2NC8yNTIxNy9zeW5jX2ludGVncmF0aW9uX3Rlc3RzIG9uIFdpbmRvd3MtMTAtMTUwNjMvVTNsdVkwRjFkR2hVWlhOMExsUnZhMlZ1Ulhod2FYSjUMCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

If the culprit above is wrong, please file a bug using this link and hit submit:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Analyzer%20-%20Wrong%20result%20for%20SyncAuthTest.TokenExpiry&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVypAELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCJuY2hyb21pdW0ud2luL1dpbjEwIFRlc3RzIHg2NC8yNTIxNy9zeW5jX2ludGVncmF0aW9uX3Rlc3RzIG9uIFdpbmRvd3MtMTAtMTUwNjMvVTNsdVkwRjFkR2hVWlhOMExsUnZhMlZ1Ulhod2FYSjUMCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).

+droger@ as the CL FindIt regards as culprit was "Enable Dice M3 in fieldtrial_testing_config"

+jcivelli@ in case this may be related to SimpleURLLoader migration

+treib@ for sync expertise

Example failure log:
[ RUN      ] SyncAuthTest.TokenExpiry
[15866:15866:0703/202626.458791:3061561761:WARNING:loopback_server.cc(638)] Loopback sync persistent state file does not exist.
Xlib:  extension "RANDR" missing on display ":99".
[15866:15866:0703/202626.613051:WARNING:password_store_factory.cc(250)] Using basic (unencrypted) store for password storage. See https://chromium.googlesource.com/chromium/src/+/master/docs/linux_password_storage.md for more information about password storage options.
[15866:15914:0703/202626.622967:ERROR:bus.cc(394)] Failed to connect to the bus: Could not parse server address: Unknown address type (examples of valid types are "tcp" and on UNIX "unix")
(sync_integration_tests:15866): LIBDBUSMENU-GLIB-WARNING **: Unable to get session bus: Unknown or unsupported transport 'disabled' for address 'disabled:'
[15866:15866:0703/202626.698932:WARNING:password_store_factory.cc(250)] Using basic (unencrypted) store for password storage. See https://chromium.googlesource.com/chromium/src/+/master/docs/linux_password_storage.md for more information about password storage options.
[15866:15866:0703/202626.745288:WARNING:password_store_factory.cc(250)] Using basic (unencrypted) store for password storage. See https://chromium.googlesource.com/chromium/src/+/master/docs/linux_password_storage.md for more information about password storage options.
[15866:15866:0703/202626.810650:ERROR:account_info_fetcher.cc(74)] OnOAuthError
[15866:15866:0703/202626.810670:WARNING:account_fetcher_service.cc(322)] Failed to get UserInfo for gaia-id-user@gmail.com
[15866:15866:0703/202626.819282:ERROR:account_tracker.cc(267)] OnOAuthError
[15866:15866:0703/202626.819299:WARNING:account_tracker.cc(194)] Failed to get UserInfo for gaia-id-user@gmail.com
[15866:16008:0703/202626.833379:WARNING:sync_encryption_handler_impl.cc(970)] Nigori had empty encryption keybag.
[15866:16008:0703/202626.834856:WARNING:sync_encryption_handler_impl.cc(970)] Nigori had empty encryption keybag.
[15866:16008:0703/202626.840569:WARNING:sync_encryption_handler_impl.cc(346)] Ignoring new implicit passphrase. Keystore migration already performed.
[15866:16008:0703/202631.856889:WARNING:syncer_proto_util.cc(338)] Error posting from syncer: Response Code (bogus on error): 401 Content-Length (bogus on error): 0 Server Status: SYNC_AUTH_ERROR
[15866:15866:0703/202631.859133:WARNING:profile_downloader.cc(333)] ProfileDownloader: token request using refresh token failed:Service unavailable; try again later.
BrowserTestBase received signal: Terminated. Backtrace:
#0 0x0000039895fc base::debug::StackTrace::StackTrace()
#1 0x000007b016c5 content::(anonymous namespace)::DumpStackTraceSignalHandler()
#2 0x7f161cf44cb0 <unknown>
#3 0x7f161cffec9d __poll
#4 0x7f1622491fe4 <unknown>
#5 0x7f16224920ec g_main_context_iteration
#6 0x00000391e142 base::MessagePumpGlib::Run()
#7 0x000003939785 base::RunLoop::Run()
#8 0x0000037d6fb1 StatusChangeChecker::StartBlockingWait()
#9 0x0000037d6ebb StatusChangeChecker::Wait()
#10 0x000001b89514 SyncAuthTest_TokenExpiry_Test::RunTestOnMainThread()
#11 0x000007b00e38 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#12 0x000003c119eb ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#13 0x000003c106c1 ChromeBrowserMainParts::PreMainMessageLoopRun()
#14 0x0000026780f1 content::BrowserMainLoop::PreMainMessageLoopRun()
#15 0x0000029df267 content::StartupTaskRunner::RunAllTasksNow()
#16 0x0000026767bd content::BrowserMainLoop::CreateStartupTasks()
#17 0x00000267a915 content::BrowserMainRunnerImpl::Initialize()
#18 0x00000267462c content::BrowserMain()
#19 0x0000037aa001 content::ContentMainRunnerImpl::Run()
#20 0x00000500755a service_manager::Main()
#21 0x0000037a8084 content::ContentMain()
#22 0x000007b00a2d content::BrowserTestBase::SetUp()
#23 0x0000037e644b InProcessBrowserTest::SetUp()
#24 0x0000037db9ad SyncTest::SetUp()
#25 0x000001cf418d testing::Test::Run()
#26 0x000001cf4d10 testing::TestInfo::Run()
#27 0x000001cf5227 testing::TestCase::Run()
#28 0x000001d00b07 testing::internal::UnitTestImpl::RunAllTests()
#29 0x000001d0067d testing::UnitTest::Run()
#30 0x0000038ef201 base::TestSuite::Run()
#31 0x0000037e5772 ChromeTestSuiteRunner::RunTestSuite()
#32 0x000007b03f4f content::LaunchTests()
#33 0x0000037e5c05 LaunchChromeTests()
#34 0x0000037bcebe main
#35 0x7f161cf2ff45 __libc_start_main
#36 0x000001b4b02a _start
[137/330] SyncAuthTest.TokenExpiry (TIMED OUT)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1ecfdc758070d74a1336264ac29ed377e9154805

commit 1ecfdc758070d74a1336264ac29ed377e9154805
Author: Pavol Marko <pmarko@chromium.org>
Date: Wed Jul 04 13:27:17 2018

Disable SyncAuthTest due to flakiness

TBR=treib@chromium.org

Bug: 860200
Change-Id: Id0a9b33a4dc185ab7834fbf0dbf478ae3d6e8d26
Reviewed-on: https://chromium-review.googlesource.com/1126101
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572548}
[modify] https://crrev.com/1ecfdc758070d74a1336264ac29ed377e9154805/chrome/browser/sync/test/integration/sync_auth_test.cc

Disabled the test for now because I don't understand if the flakes are related to the possible culprit and I didn't feel confident reverting a CL that's 10 days old.

droger@, mind checking if the feature you were enabling in https://crrev.com/569640 could be causing this?

The CL was indeed related to authentication, so I'm not ruling out the possibility. However I don't quite see how it would impact this test. I'll try to repro.