See https://ci.chromium.org/buildbot/chromium.chromiumos/linux-chromeos-dbg/5379 and log
[ RUN      ] KioskAppManagerTest.BadApp
[16828:16828:0425/214104.145236:WARNING:chrome_browser_main_chromeos.cc(640)] Running as stub user with profile dir: test-user
[16828:17136:0425/214104.503586:ERROR:logging_chrome.cc(218)] Unable to create symlink /b/s/w/itzPaaJU/.org.chromium.Chromium.r2dlAV/dfygb7a/test-user/chrome_debug.log pointing at /b/s/w/itzPaaJU/.org.chromium.Chromium.r2dlAV/dfygb7a/test-user/chrome_debug_20180425-214104: No such file or directory (2)
[16828:17135:0425/214104.813100:ERROR:account_manager.cc(37)] Failed to read tokens file
[16828:16828:0425/214104.910711:WARNING:user_session_manager.cc(1086)] Attempting to save user password for non enterprise user.
[16828:16828:0425/214105.072565:WARNING:app_launch_info.cc(280)] Override URL passed for Web Store should not contain a port.  Removing it.
[16828:16828:0425/214105.940019:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[16828:16828:0425/214105.941597:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[16828:16828:0425/214105.944472:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[16828:16828:0425/214105.944935:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[1:1:0425/214106.170317:WARNING:app_launch_info.cc(280)] Override URL passed for Web Store should not contain a port.  Removing it.
[1:1:0425/214106.188917:WARNING:app_launch_info.cc(280)] Override URL passed for Web Store should not contain a port.  Removing it.
[16828:17271:0425/214106.718323:WARNING:embedded_test_server.cc(229)] Request not handled. Returning 404: /update_check.xml?os=cros&arch=x64&os_arch=x86_64&nacl_arch=x86-64&prod=chromiumcrx&prodchannel=&prodversion=68.0.3410.0&lang=en-US&acceptformat=crx2,crx3&x=id%3Dunknown_app%26v%3D0.0.0.0%26installedby%3Dpolicy%26uc
[16828:17271:0425/214106.719401:WARNING:embedded_test_server.cc(229)] Request not handled. Returning 404: /chromeos/app_mode/webstore/inlineinstall/detail/unknown_app
[16828:16828:0425/214106.721378:ERROR:external_cache_impl.cc(213)] ExternalCacheImpl failed to download extension unknown_app, error 1
../../chrome/browser/chromeos/app_mode/kiosk_app_manager_browsertest.cc:618: Failure
Expected equality of these values:
  ""
  GetAppIds()
    Which is: "unknown_app"
Stack trace:
#0 0x000003c74320 StackTraceGetter::CurrentStackTrace()
#1 0x000003c94e57 testing::internal::UnitTestImpl::CurrentOsStackTraceExceptTop()
#2 0x000003c942b2 testing::internal::AssertHelper::operator=()
#3 0x00000253adaf chromeos::KioskAppManagerTest_BadApp_Test::RunTestOnMainThread()
#4 0x00000738c36c content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#5 0x0000009d87fd _ZN4base8internal13FunctorTraitsIM25RenderViewContextMenuBaseFvvEvE6InvokeIP21RenderViewContextMenuJEEEvS4_OT_DpOT0_
#6 0x0000009d8744 _ZN4base8internal12InvokeHelperILb0EvE8MakeItSoIM25RenderViewContextMenuBaseFvvEJP21RenderViewContextMenuEEEvOT_DpOT0_
#7 0x00000738e625 _ZN4base8internal7InvokerINS0_9BindStateIMN7content15BrowserTestBaseEFvvEJNS0_17UnretainedWrapperIS4_EEEEEFvvEE7RunImplIRKS6_RKNSt3__15tupleIJS8_EEEJLm0EEEEvOT_OT0_NSF_16integer_sequenceImJXspT1_EEEE
#8 0x00000738e56c _ZN4base8internal7InvokerINS0_9BindStateIMN7content15BrowserTestBaseEFvvEJNS0_17UnretainedWrapperIS4_EEEEEFvvEE3RunEPNS0_13BindStateBaseE
#9 0x0000009ca9cd _ZNKR4base17RepeatingCallbackIFvvEE3RunEv
#10 0x00000634df08 ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#11 0x00000634c0de ChromeBrowserMainParts::PreMainMessageLoopRun()
#12 0x00000319f58d chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()
#13 0x7f73be5a496c content::BrowserMainLoop::PreMainMessageLoopRun()
#14 0x7f73bd30e72d _ZN4base8internal13FunctorTraitsIMN7content12ChildProcessEFvvEvE6InvokeIPS3_JEEEvS5_OT_DpOT0_
#15 0x7f73bd30e674 _ZN4base8internal12InvokeHelperILb0EvE8MakeItSoIMN7content12ChildProcessEFvvEJPS5_EEEvOT_DpOT0_
#16 0x7f73be5aa265 _ZN4base8internal7InvokerINS0_9BindStateIMN7content15BrowserMainLoopEFivEJNS0_17UnretainedWrapperIS4_EEEEEFivEE7RunImplIRKS6_RKNSt3__15tupleIJS8_EEEJLm0EEEEiOT_OT0_NSF_16integer_sequenceImJXspT1_EEEE
#17 0x7f73be5aa1ac _ZN4base8internal7InvokerINS0_9BindStateIMN7content15BrowserMainLoopEFivEJNS0_17UnretainedWrapperIS4_EEEEEFivEE3RunEPNS0_13BindStateBaseE
#18 0x7f73bd217bed _ZNKR4base17RepeatingCallbackIFvvEE3RunEv
#19 0x7f73bf35011d content::StartupTaskRunner::RunAllTasksNow()
#20 0x7f73be5a0abf content::BrowserMainLoop::CreateStartupTasks()
#21 0x7f73be5ac4b3 content::BrowserMainRunnerImpl::Initialize()
#22 0x7f73be598efe content::BrowserMain()
#23 0x7f73c0540f9e content::RunNamedProcessTypeMain()
#24 0x7f73c054464a content::ContentMainRunnerImpl::Run()
#25 0x7f73c0537e25 content::ContentServiceManagerMainDelegate::RunEmbedderProcess()
#26 0x7f73b9846693 service_manager::Main()
#27 0x7f73c053df7b content::ContentMain()
#28 0x00000738b908 content::BrowserTestBase::SetUp()
#29 0x000006253f43 InProcessBrowserTest::SetUp()
#30 0x00000254cadf chromeos::KioskAppManagerTest::SetUp()
#31 0x00000095524e _ZN7testing8internal12InvokeHelperIRKNSt3__16vectorIPN12user_manager4UserENS2_9allocatorIS6_EEEENS2_5tupleIJEEEE12InvokeMethodIN8chromeos15MockUserManagerEMSH_KFSB_vEEESB_PT_T0_RKSD_
[16828:17158:0425/214107.336070:WARNING:discardable_shared_memory_manager.cc(431)] Some MojoDiscardableSharedMemoryManagerImpls are still alive. They will be leaked.
[16828:17285:0425/214107.517249:WARNING:internal_linux.cc(64)] Failed to read /proc/17161/stat
[16828:16828:0425/214107.518848:WARNING:url_request_context_getter.cc(43)] URLRequestContextGetter leaking due to no owning thread.
[16828:16828:0425/214107.549018:WARNING:pref_notifier_impl.cc(23)] Pref observer found at shutdown.
[16828:16828:0425/214107.549110:WARNING:pref_notifier_impl.cc(23)] Pref observer found at shutdown.
[  FAILED  ] KioskAppManagerTest.BadApp, where TypeParam =  and GetParam() =  (7106 ms)

Findit seems to think it's in the range https://chromium.googlesource.com/chromium/src/+log/f8f8f9584ddbcf7e4c4e226f7865fad91cacd7bb..c30b43e9f3f6e21fc670e6ba695f7c183050e433?pretty=fuller

Maybe https://chromium-review.googlesource.com/c/chromium/src/+/1024077?

Mark: could you have a look and check if it's related to your CL?