Looks like caused by https://chromium-review.googlesource.com/c/chromium/src/+/892419

Since #1 is meant to reduce flakiness of some other tests, I'll disable the flaky ones now on ChromeOS.

Disabling them here https://chromium-review.googlesource.com/c/chromium/src/+/907553

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/21a534ef80dc7ab11a42747f2601bdc48a740ba8

commit 21a534ef80dc7ab11a42747f2601bdc48a740ba8
Author: Olga Sharonova <olka@chromium.org>
Date: Thu Feb 08 11:00:47 2018

Disable DownloadNotificationTest.DiscardDangerousFile/DownloadDangerousFile on ChromeOS

Flaky.

TBR=estade@chromium.org,peter@chromium.org

Bug: 810302
Change-Id: I01b370c421a4c1861c1c335eb304d520d267fa18
Reviewed-on: https://chromium-review.googlesource.com/907553
Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535350}
[modify] https://crrev.com/21a534ef80dc7ab11a42747f2601bdc48a740ba8/chrome/browser/download/notification/download_notification_browsertest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/28322d7a97eb3e7d757e4dbca01681a2230f1853

commit 28322d7a97eb3e7d757e4dbca01681a2230f1853
Author: Evan Stade <estade@chromium.org>
Date: Fri Mar 02 00:06:26 2018

Download notification tests: browser tests => interactive ui tests

Also, re-enable the tests which had been identified as flaky.

Bug:  810738 , 810302
Change-Id: Ieafb9a09450f4a3a61f2e1e14d40dc93366a875d
Reviewed-on: https://chromium-review.googlesource.com/938362
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540358}
[rename] https://crrev.com/28322d7a97eb3e7d757e4dbca01681a2230f1853/chrome/browser/download/notification/download_notification_interactive_uitest.cc
[modify] https://crrev.com/28322d7a97eb3e7d757e4dbca01681a2230f1853/chrome/test/BUILD.gn

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1172c37b4ad4ed555b59a3010290988beee1eec1

commit 1172c37b4ad4ed555b59a3010290988beee1eec1
Author: Greg Thompson <grt@chromium.org>
Date: Fri Mar 02 12:27:28 2018

Disable DownloadNotificationTest.DiscardDangerousFile.

Times out fairly consistently.

BUG=810302
TBR=estade@chromium.org

Change-Id: I345f7d1ffd6d5e5dedc18cb3aaf4ef669200b59a
Reviewed-on: https://chromium-review.googlesource.com/945791
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540501}
[modify] https://crrev.com/1172c37b4ad4ed555b59a3010290988beee1eec1/chrome/browser/download/notification/download_notification_interactive_uitest.cc

DownloadDangerousFile and DownloadImageFile are flaky as well:

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=interactive_ui_tests&tests=DownloadNotificationTest.*

 Issue 818166  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9725effd4258d6e96b1ea52aae1f73ab084ec39e

commit 9725effd4258d6e96b1ea52aae1f73ab084ec39e
Author: Greg Thompson <grt@chromium.org>
Date: Fri Mar 02 14:14:53 2018

Disable DownloadNotificationTest.Download{Dangerous,Image}File due to flakes.

BUG=810302
TBR=grt@chromium.org

Change-Id: Iabaf1bb1466db928e48f80eed48c0b4f7b3fab04
Reviewed-on: https://chromium-review.googlesource.com/946094
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540512}
[modify] https://crrev.com/9725effd4258d6e96b1ea52aae1f73ab084ec39e/chrome/browser/download/notification/download_notification_interactive_uitest.cc

dang, still flaky and still not able to repro locally or on CQ. For posterity:

[ RUN      ] DownloadNotificationTest.DownloadDangerousFile
[4843:4843:0302/042315.208582:WARNING:chrome_browser_main_chromeos.cc(613)] Running as stub user with profile dir: test-user
[4843:4862:0302/042315.601098:ERROR:logging_chrome.cc(218)] Unable to create symlink /b/s/w/itg04Fzc/.org.chromium.Chromium.kg1XVh/dXd5TpN/test-user/chrome_debug.log pointing at /b/s/w/itg04Fzc/.org.chromium.Chromium.kg1XVh/dXd5TpN/test-user/chrome_debug_20180302-042315: No such file or directory (2)
[4843:4843:0302/042315.804412:WARNING:user_session_manager.cc(1060)] Attempting to save user password for non enterprise user.
[4843:4843:0302/042316.230493:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[4843:4843:0302/042316.231576:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[4843:4843:0302/042316.233418:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[4843:4843:0302/042316.233827:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[4843:4843:0302/042316.494386:ERROR:wallpaper_controller.cc(1263)] User is ephemeral or guest! Fallback to default wallpaper.
[4843:4865:0302/042316.954939:WARNING:simple_synchronous_entry.cc(1255)] Could not open platform files for entry.
[4843:4843:0302/042317.021328:ERROR:in_progress_cache_impl.cc(189)] Cache is not initialized, cannot RetrieveEntry.
[4843:4843:0302/042317.021389:ERROR:in_progress_cache_impl.cc(173)] Cache is not initialized, cannot AddOrReplaceEntry.
[4843:4843:0302/042317.022035:ERROR:in_progress_cache_impl.cc(189)] Cache is not initialized, cannot RetrieveEntry.
BrowserTestBase received signal: Terminated. Backtrace:
#0 0x7fc9c9fc53dd base::debug::StackTrace::StackTrace()
#1 0x7fc9c9fc3a0c base::debug::StackTrace::StackTrace()
#2 0x000004942d54 content::(anonymous namespace)::DumpStackTraceSignalHandler()
#3 0x7fc9ab88bcb0 <unknown>
#4 0x7fc9ab953693 epoll_wait
#5 0x7fc9ca3156d1 epoll_dispatch
#6 0x7fc9ca308cc5 event_base_loop
#7 0x7fc9ca07121a base::MessagePumpLibevent::Run()
#8 0x7fc9ca06b3da base::MessageLoop::Run()
#9 0x7fc9ca120e62 base::RunLoop::Run()
#10 0x000000a439a3 (anonymous namespace)::WaitForDownloadNotificationForDisplayService()
#11 0x000000a459c5 DownloadNotificationTest::WaitForDownloadNotification()
#12 0x000000a44d39 DownloadNotificationTest::CreateDownloadForBrowserAndURL()
#13 0x000000a33bad DownloadNotificationTest_DownloadDangerousFile_Test::RunTestOnMainThread()
#14 0x000004942804 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#15 0x00000067da0d _ZN4base8internal13FunctorTraitsIMN25WebViewImeInteractiveTest30CompositionRangeUpdateObserverEFvvEvE6InvokeIPS3_JEEEvS5_OT_DpOT0_
#16 0x00000067d954 _ZN4base8internal12InvokeHelperILb0EvE8MakeItSoIMN26WebViewInteractiveTestBase20PopupCreatedObserverEFvvEJPS5_EEEvOT_DpOT0_
#17 0x000004944e05 _ZN4base8internal7InvokerINS0_9BindStateIMN7content15BrowserTestBaseEFvvEJNS0_17UnretainedWrapperIS4_EEEEEFvvEE7RunImplIRKS6_RKNSt3__15tupleIJS8_EEEJLm0EEEEvOT_OT0_NSF_16integer_sequenceImJXspT1_EEEE
#18 0x000004944d4c _ZN4base8internal7InvokerINS0_9BindStateIMN7content15BrowserTestBaseEFvvEJNS0_17UnretainedWrapperIS4_EEEEEFvvEE3RunEPNS0_13BindStateBaseE
#19 0x0000006d3e4d _ZNKR4base17RepeatingCallbackIFvvEE3RunEv
#20 0x000003b0a87d ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#21 0x000003b08a8e ChromeBrowserMainParts::PreMainMessageLoopRun()
#22 0x0000029491c1 chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()
#23 0x7fc9c535625c content::BrowserMainLoop::PreMainMessageLoopRun()
#24 0x7fc9c41d903d _ZN4base8internal13FunctorTraitsIMN7content12ChildProcessEFvvEvE6InvokeIPS3_JEEEvS5_OT_DpOT0_
#25 0x7fc9c41d8f84 _ZN4base8internal12InvokeHelperILb0EvE8MakeItSoIMN7content12ChildProcessEFvvEJPS5_EEEvOT_DpOT0_
#26 0x7fc9c535d7d5 _ZN4base8internal7InvokerINS0_9BindStateIMN7content15BrowserMainLoopEFivEJNS0_17UnretainedWrapperIS4_EEEEEFivEE7RunImplIRKS6_RKNSt3__15tupleIJS8_EEEJLm0EEEEiOT_OT0_NSF_16integer_sequenceImJXspT1_EEEE
#27 0x7fc9c535d71c _ZN4base8internal7InvokerINS0_9BindStateIMN7content15BrowserMainLoopEFivEJNS0_17UnretainedWrapperIS4_EEEEEFivEE3RunEPNS0_13BindStateBaseE
#28 0x7fc9c40ef2fd _ZNKR4base17RepeatingCallbackIFvvEE3RunEv
#29 0x7fc9c60efb7d content::StartupTaskRunner::RunAllTasksNow()
#30 0x7fc9c5351b30 content::BrowserMainLoop::CreateStartupTasks()
#31 0x7fc9c535fa4f content::BrowserMainRunnerImpl::Initialize()

Findit identified the culprit r540358 with confidence 100.0% in the config "chromium.chromiumos / linux-chromeos-dbg"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyuAELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKBAWNocm9taXVtLmNocm9taXVtb3MvbGludXgtY2hyb21lb3MtZGJnLzQ1MTgvaW50ZXJhY3RpdmVfdWlfdGVzdHMvUkc5M2JteHZZV1JPYjNScFptbGpZWFJwYjI1VVpYTjBMa1J2ZDI1c2IyRmtSR0Z1WjJWeWIzVnpSbWxzWlE9PQwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAIM

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).
Flake Analyzer is in alpha version.
Feedback is welcome using component Tools>Test>FindIt>Flakiness !

Findit identified the culprit r540358 with confidence 100.0% in the config "chromium.chromiumos / linux-chromeos-dbg"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyuAELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKBAWNocm9taXVtLmNocm9taXVtb3MvbGludXgtY2hyb21lb3MtZGJnLzQ1MTgvaW50ZXJhY3RpdmVfdWlfdGVzdHMvUkc5M2JteHZZV1JPYjNScFptbGpZWFJwYjI1VVpYTjBMa1J2ZDI1c2IyRmtSR0Z1WjJWeWIzVnpSbWxzWlE9PQwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAMM

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).
Flake Analyzer is in alpha version.
Feedback is welcome using component Tools>Test>FindIt>Flakiness !

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0816757edbe7e9d5ef98e6b7d1585d82e41a5844

commit 0816757edbe7e9d5ef98e6b7d1585d82e41a5844
Author: Evan Stade <estade@chromium.org>
Date: Fri Mar 09 03:24:16 2018

[Chrome OS] Fixes for download shutdown path.

a) if a dangerous download is cancelled, don't try to update it (with
a potentially half-destroyed profile). Like other notification types,
just close and return.

b) make the download service depend on the NotificationDisplayService.

Bug: 810302,  810738 ,  578868 
Change-Id: Ibc3080510e128954e233485606d8de520b72a97c
Reviewed-on: https://chromium-review.googlesource.com/956087
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542010}
[modify] https://crrev.com/0816757edbe7e9d5ef98e6b7d1585d82e41a5844/chrome/browser/download/download_core_service_factory.cc
[modify] https://crrev.com/0816757edbe7e9d5ef98e6b7d1585d82e41a5844/chrome/browser/download/notification/download_item_notification.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/024f0186e35daffb4faae0c4d69423adbf8d6385

commit 024f0186e35daffb4faae0c4d69423adbf8d6385
Author: Evan Stade <estade@chromium.org>
Date: Wed Mar 14 21:25:21 2018

Re-enable DiscardDangerousFile in hopes that it is not flaky.

0816757edbe7e9d5ef98e6b fixed a real issue which might be the source
of the flake here.

Bug: 810302
Change-Id: I38630563e11944deec2dcd71dc89a01eabdfdd74
Reviewed-on: https://chromium-review.googlesource.com/957806
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543199}
[modify] https://crrev.com/024f0186e35daffb4faae0c4d69423adbf8d6385/chrome/browser/download/notification/download_notification_interactive_uitest.cc

I don't see any failures after re-enabling, but it seems like the test is being skipped? Not sure if I'm holding the flakiness dash right.

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=interactive_ui_tests&tests=DownloadNotificationTest.DiscardDangerousFile

DownloadDangerousFile just flaked on https://ci.chromium.org/buildbot/chromium.chromiumos/linux-chromeos-dbg/4731 (and twice previously in last 24 hours).

Ah, I was pretty confused because I got the test name wrong in 024f0186e35daffb4faae0c4d69423adbf8d6385 --- I said I was re-enabling DiscardDangerousFile but it was actually DownloadDangerousFile. The test has already been re-disabled, mystery remains unsolved.

 Issue 822470  has been merged into this issue.