Log:

UserManagerBrowserTest.UserManagerTest (run #1):
[ RUN      ] UserManagerBrowserTest.UserManagerTest
Xlib:  extension "RANDR" missing on display ":9".
Xlib:  extension "RANDR" missing on display ":9".
[24596:24596:0519/082455:WARNING:audio_manager.cc(297)] Multiple instances of AudioManager detected
[24596:24596:0519/082455:WARNING:audio_manager.cc(271)] Multiple instances of AudioManager detected
[24596:24596:0519/082455: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.
[24596:24596:0519/082456:WARNING:CONSOLE(1)] "This file is deprecated. Please use `iron-flex-layout/iron-flex-layout-classes.html`, and one of the specific dom-modules instead", source: chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-shadow-flex-layout-extracted.js (1)
[24596:24596:0519/082456:WARNING:CONSOLE(1)] "This file is deprecated. Please use `iron-flex-layout/iron-flex-layout-classes.html`, and one of the specific dom-modules instead", source: chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout-extracted.js (1)
[24596:24596:0519/082457:WARNING:CONSOLE(0)] "/deep/ combinator is deprecated. See https://www.chromestatus.com/features/6750456638341120 for more details.", source:  (0)
[24596:24596:0519/082457:INFO:CONSOLE(1196)] "Running TestCase UserManagerBrowserTest.UserManagerTest", source: test_api.js (1196)
[24596:24596:0519/082459:ERROR:CONSOLE(44)] "Mocha test failed: ImportSupervisedUserTests Can import supervised user
Error: Script error. (:0)
", source: mocha_adapter.js (44)
[24596:24596:0519/082500:INFO:CONSOLE(121)] "Page load time: 819 ms", source: polymer_browser_test_base.js (121)
[24596:24596:0519/082500:INFO:CONSOLE(123)] "Test run time: 2826 ms", source: polymer_browser_test_base.js (123)
[24596:24596:0519/082500:INFO:CONSOLE(125)] "Total time: 3645 ms", source: polymer_browser_test_base.js (125)
[24596:24596:0519/082500:ERROR:web_ui_test_handler.cc(76)] Test Errors: 1/28 tests had failed assertions.
[24596:24596:0519/082500:ERROR:web_ui_browser_test.cc(462)] Encountered javascript console error(s)
gen/chrome/test/data/webui/md_user_manager/user_manager_browsertest-gen.cc:43: Failure
Value of: RunJavascriptTestF( true, "UserManagerBrowserTest", "UserManagerTest")
  Actual: false
Expected: true
[  FAILED  ] UserManagerBrowserTest.UserManagerTest, where TypeParam =  and GetParam() =  (4300 ms)

@mahmadi, git blame indicates you touched the code most recently, assign it to you for triage.

Sheriffs, please disable this test.  It's preventing CLs from landing via the commit queue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1273112816d19f19f1142dfda1ed95b9e8a6de1f

commit 1273112816d19f19f1142dfda1ed95b9e8a6de1f
Author: spqchan <spqchan@chromium.org>
Date: Thu May 19 20:10:09 2016

Disabled flaky UserManagerBrowserTest.UserManagerTest

BUG= 613191 

TBR=sky@chromium.org

Review-Url: https://codereview.chromium.org/1996093002
Cr-Commit-Position: refs/heads/master@{#394848}

[modify] https://crrev.com/1273112816d19f19f1142dfda1ed95b9e8a6de1f/chrome/test/data/webui/md_user_manager/user_manager_browsertest.js

Issue 613309 has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6d1a8b812ce52ce95301914cb328434ac61ab204

commit 6d1a8b812ce52ce95301914cb328434ac61ab204
Author: Michael Giuffrida <michaelpg@chromium.org>
Date: Fri May 20 05:40:31 2016

Fix UserManagerBrowserTest

Suppresses the focus() call if the element is not shown, and adds
some time between tests to finish asynchronous microtasks.

CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation
BUG= 613191 
R=dbeam@chromium.org, mahmadi@chromium.org
TBR=dbeam@chromium.org

Review URL: https://codereview.chromium.org/1995043003 .

Cr-Commit-Position: refs/heads/master@{#395006}

[modify] https://crrev.com/6d1a8b812ce52ce95301914cb328434ac61ab204/chrome/browser/resources/md_user_manager/import_supervised_user.js
[modify] https://crrev.com/6d1a8b812ce52ce95301914cb328434ac61ab204/chrome/test/data/webui/md_user_manager/import_supervised_user_tests.js

I don't think this is fixed.

Here it failed in the logs but probably succeeded on retry:
https://build.chromium.org/p/chromium.win/builders/Win7%20(32)%20Tests/builds/5820/steps/browser_tests%20on%20Windows-7-SP1/logs/stdio

Here it turned the bot red:
https://build.chromium.org/p/chromium.fyi/builders/Site%20Isolation%20Win/builds/14834

Similar error as comment 1:

[ RUN      ] UserManagerBrowserTest.UserManagerTest
[5736:1988:0602/074153:WARNING:chrome_browser_main_win.cc(420)] Command line too long for RegisterApplicationRestart
[4572:2664:0602/074153:ERROR:singleton_hwnd.cc(34)] Cannot create windows on non-UI thread!
[5280:4192:0602/074153:ERROR:singleton_hwnd.cc(34)] Cannot create windows on non-UI thread!
[5736:1248:0602/074153:ERROR:process_win.cc(134)] Unable to terminate process: Access is denied. (0x5)
[5736:1988:0602/074154:WARNING:CONSOLE(1)] "This file is deprecated. Please use `iron-flex-layout/iron-flex-layout-classes.html`, and one of the specific dom-modules instead", source: chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-shadow-flex-layout-extracted.js (1)
[5736:1988:0602/074154:WARNING:CONSOLE(1)] "This file is deprecated. Please use `iron-flex-layout/iron-flex-layout-classes.html`, and one of the specific dom-modules instead", source: chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout-extracted.js (1)
[5736:1988:0602/074154:WARNING:CONSOLE(0)] "/deep/ combinator is deprecated. See https://www.chromestatus.com/features/6750456638341120 for more details.", source:  (0)
[5736:1988:0602/074154:INFO:CONSOLE(1196)] "Running TestCase UserManagerBrowserTest.UserManagerTest", source: test_api.js (1196)
[5736:1988:0602/074157:ERROR:CONSOLE(44)] "Mocha test failed: ImportSupervisedUserTests "after each" hook
Error: Uncaught TypeError: Cannot read property 'classList' of null (user_manager.js:4789)
", source: mocha_adapter.js (44)
[5736:1988:0602/074157:INFO:CONSOLE(121)] "Page load time: 990 ms", source: polymer_browser_test_base.js (121)
[5736:1988:0602/074157:INFO:CONSOLE(123)] "Test run time: 2928 ms", source: polymer_browser_test_base.js (123)
[5736:1988:0602/074157:INFO:CONSOLE(125)] "Total time: 3919 ms", source: polymer_browser_test_base.js (125)
[5736:1988:0602/074157:ERROR:web_ui_test_handler.cc(76)] Test Errors: 1/27 tests had failed assertions.
[5736:1988:0602/074157:ERROR:web_ui_browser_test.cc(462)] Encountered javascript console error(s)
  Actual: false
Expected: true
[  FAILED  ] UserManagerBrowserTest.UserManagerTest, where TypeParam =  and GetParam() =  (4522 ms)

Verified https://codereview.chromium.org/2036763003/ eliminated the flakiness on Ubuntu 14.04. Please re-open if the problem persists.