This test is very flaky and my patch just failed on it. Here's a list of the flakes:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=UserAddingScreenTest.ScreenVisibility

The first flake was at 567829 (6/15/2018) which was a patch in this area: https://chromium-review.googlesource.com/c/chromium/src/+/1102035. The test actually flaked when trying to land this patch.

Here's the failing stacktrace:
[9970:9970:0615/173403.883497:FATAL:accelerator_manager.cc(32)] Check failed: !base::ContainsValue(targets, target). Registering the same target multiple times
#0 0x00000497d19c base::debug::StackTrace::StackTrace()
#1 0x0000048fed8b logging::LogMessage::~LogMessage()
#2 0x000005853f09 ui::AcceleratorManager::Register()
#3 0x000001c9fe42 chromeos::WebUILoginView::WebUILoginView()
#4 0x000001c96ecc chromeos::LoginDisplayHostWebUI::InitLoginWindowAndView()
#5 0x000001c95675 chromeos::LoginDisplayHostWebUI::OnStartUserAdding()
#6 0x000001c9ea77 chromeos::(anonymous namespace)::UserAddingScreenImpl::Start()
#7 0x0000015dc8bc chromeos::UserAddingScreenTest_ScreenVisibility_Test::RunTestOnMainThread()
#8 0x000004f40797 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#9 0x000004a403fe ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#10 0x000004a3f1fa ChromeBrowserMainParts::PreMainMessageLoopRun()
#11 0x000001b406fc chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()

Detected 10 new flakes for test/step "UserAddingScreenTest.ScreenVisibility". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyMAsSBUZsYWtlIiVVc2VyQWRkaW5nU2NyZWVuVGVzdC5TY3JlZW5WaXNpYmlsaXR5DA. This message was posted automatically by the chromium-try-flakes app.

Detected 18 new flakes for test/step "UserAddingScreenTest.ScreenVisibility". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyMAsSBUZsYWtlIiVVc2VyQWRkaW5nU2NyZWVuVGVzdC5TY3JlZW5WaXNpYmlsaXR5DA. This message was posted automatically by the chromium-try-flakes app.

Revert at: https://chromium-review.googlesource.com/c/chromium/src/+/1103957

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/47a732b081e2ee058b57be3726a46931b507fae1

commit 47a732b081e2ee058b57be3726a46931b507fae1
Author: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Date: Mon Jun 18 05:56:48 2018

Revert "Support feedback keyboard shortcut in views login screen."

This reverts commit b02780531c2986c564a369263e55fc74596d74b9.

Reason for revert: UserAddingScreenTest.ScreenVisibility has been
flaky since this landed:

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=UserAddingScreenTest.ScreenVisibility

[28936:28936:0617/213745.316026:FATAL:accelerator_manager.cc(32)] Check failed: !base::ContainsValue(targets, target). Registering the same target multiple times
#0 0x000004980f5c base::debug::StackTrace::StackTrace()
#1 0x000004902b4b logging::LogMessage::~LogMessage()
#2 0x000005857d49 ui::AcceleratorManager::Register()
#3 0x000001ca3442 chromeos::WebUILoginView::WebUILoginView()
#4 0x000001c9a51c chromeos::LoginDisplayHostWebUI::InitLoginWindowAndView()
#5 0x000001c98cc5 chromeos::LoginDisplayHostWebUI::OnStartUserAdding()
#6 0x000001ca20c7 chromeos::(anonymous namespace)::UserAddingScreenImpl::Start()
#7 0x0000015dfd1c chromeos::UserAddingScreenTest_ScreenVisibility_Test::RunTestOnMainThread()
#8 0x000004f449b7 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()

Original change's description:
> Support feedback keyboard shortcut in views login screen.
>
> This CL supports alt+shift+i to open the feedback dialog in views login
> screen.
>
> Bug:  852242 
> Change-Id: I9a285c6929fd2302d22f90fbefbb32bcd814359d
> Reviewed-on: https://chromium-review.googlesource.com/1102035
> Reviewed-by: Tom Sepez <tsepez@chromium.org>
> Reviewed-by: Jacob Dufault <jdufault@chromium.org>
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Commit-Queue: Xiaoyin Hu <xiaoyinh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#567829}

TBR=xiyuan@chromium.org,tsepez@chromium.org,jdufault@chromium.org,xiaoyinh@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug:  852242 ,  853445 
Change-Id: I21afadf31af3c9ae8e04a30f6021b928efde43f8
Reviewed-on: https://chromium-review.googlesource.com/1103957
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567935}
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/ash/login/login_screen_controller.cc
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/ash/login/login_screen_controller.h
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/ash/login/mock_login_screen_client.h
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/ash/login/ui/lock_contents_view.cc
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/ash/login/ui/lock_contents_view.h
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/ash/public/interfaces/login_screen.mojom
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/chromeos/login/ui/fake_login_display_host.cc
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/chromeos/login/ui/fake_login_display_host.h
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/chromeos/login/ui/login_display_host.h
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/chromeos/login/ui/login_display_host_mojo.cc
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/chromeos/login/ui/login_display_host_mojo.h
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/chromeos/login/ui/login_display_host_webui.cc
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/chromeos/login/ui/login_display_host_webui.h
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/chromeos/login/ui/mock_login_display_host.h
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/ui/ash/login_screen_client.cc
[modify] https://crrev.com/47a732b081e2ee058b57be3726a46931b507fae1/chrome/browser/ui/ash/login_screen_client.h

There have been no more flakes since the revert landed. Removing from sheriff queue.

I re-landed the CL: https://chromium-review.googlesource.com/c/chromium/src/+/1104895 and UserAddingScreenTest.ScreenVisibility seems to be no longer flaky since I landed it. So consider this as fixed.