possibly because of issue 899073?

A sample failure:

[ RUN      ] WebBluetoothTest.NavigateWithChooserCrossOrigin
[13828:13828:1204/200714.200740:WARNING:chrome_browser_main_chromeos.cc(544)] Running as stub user with profile dir: test-user
[13828:13828:1204/200714.274589:WARNING:easy_unlock_service_regular.cc(523)] EasyUnlockServiceRegular::IsInLegacyHostMode: DeviceSyncClient not ready. Returning false.
[13828:13828:1204/200714.306230:INFO:remote_commands_service.cc(38)] Fetching remote commands.
[13828:13828:1204/200714.329472:WARNING:remote_commands_service.cc(40)] Client is not registered.
[13828:13828:1204/200714.329852:INFO:remote_commands_invalidator.cc(32)] Initialize RemoteCommandsInvalidator.
[13828:13828:1204/200714.330994:INFO:remote_commands_invalidator.cc(57)] Starting RemoteCommandsInvalidator.
[13828:13828:1204/200714.331307:INFO:remote_commands_invalidator.cc(123)] RemoteCommandsInvalidator ReloadPolicyData.
[13828:13828:1204/200714.331535:INFO:remote_commands_invalidator.cc(167)] Unregister RemoteCommandsInvalidator.
[13828:13828:1204/200714.357458:WARNING:wallpaper_controller_client.cc(358)] Cannot get wallpaper files id in RemovePolicyWallpaper. This should never happen under normal circumstances.
[13828:13828:1204/200714.489275:ERROR:gpu_interface_provider.cc(87)] Not implemented reached in virtual void content::GpuInterfaceProvider::RegisterOzoneGpuInterfaces(service_manager::BinderRegistry *)
[13828:13828:1204/200714.529436:ERROR:remote_text_input_client.cc(158)] Not implemented reached in virtual void RemoteTextInputClient::OnInputMethodChanged()
[13828:13828:1204/200714.529472:ERROR:remote_text_input_client.cc(115)] Not implemented reached in virtual ui::TextInputClient::FocusReason RemoteTextInputClient::GetFocusReason() const
[13828:13828:1204/200714.529477:ERROR:remote_text_input_client.cc(200)] Not implemented reached in virtual bool RemoteTextInputClient::ShouldDoLearning()
[13828:13828:1204/200714.529492:ERROR:remote_text_input_client.cc(109)] Not implemented reached in virtual bool RemoteTextInputClient::HasCompositionText() const
[13828:13828:1204/200714.529497:ERROR:remote_text_input_client.cc(121)] Not implemented reached in virtual bool RemoteTextInputClient::GetTextRange(gfx::Range *) const
[13828:13828:1204/200714.529502:ERROR:remote_text_input_client.cc(176)] Not implemented reached in virtual void RemoteTextInputClient::EnsureCaretNotInRect(const gfx::Rect &)
[13828:13828:1204/200714.551328:ERROR:render_widget_host_view_aura.cc(1298)] Not implemented reached in virtual base::i18n::TextDirection content::RenderWidgetHostViewAura::GetTextDirection() const
[13828:14076:1204/200714.582176:WARNING:embedded_test_server.cc(239)] Request not handled. Returning 404: /favicon.ico
../../chrome/browser/web_bluetooth_browsertest.cc:205: Failure
Expected equality of these values:
  0u
    Which is: 0
  browser()->GetBubbleManager()->GetBubbleCountForTesting()
    Which is: 1
Stack trace:
#0 0x56229b02457c testing::internal::UnitTestImpl::CurrentOsStackTraceExceptTop()
#1 0x56229b023f59 testing::internal::AssertHelper::operator=()
#2 0x562299ed0d00 (anonymous namespace)::WebBluetoothTest_NavigateWithChooserCrossOrigin_Test::RunTestOnMainThread()
#3 0x56229d75a91d content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#4 0x56229d318fd5 ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#5 0x56229d317f72 ChromeBrowserMainParts::PreMainMessageLoopRun()
#6 0x56229abaa532 chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()
#7 0x56229bbd80d5 content::BrowserMainLoop::PreMainMessageLoopRun()
#8 0x56229bf5ec57 content::StartupTaskRunner::RunAllTasksNow()
#9 0x56229bbd6ee0 content::BrowserMainLoop::CreateStartupTasks()
#10 0x56229bbda0c8 content::BrowserMainRunnerImpl::Initialize()
#11 0x56229bbd52c2 content::BrowserMain()
#12 0x56229ce63bc1 content::ContentMainRunnerImpl::RunServiceManager()
#13 0x56229ce63aa9 content::ContentMainRunnerImpl::Run()
#14 0x56229ef5be56 service_manager::Main()
#15 0x56229ce62124 content::ContentMain()
#16 0x56229d75a551 content::BrowserTestBase::SetUp()
#17 0x56229d2cac44 InProcessBrowserTest::SetUp()

[13828:13828:1204/200714.942271:ERROR:http_bridge.cc(126)] Not implemented reached in virtual void syncer::HttpBridgeFactory::OnSignalReceived()
[13828:13828:1204/200714.942937:INFO:remote_commands_invalidator.cc(45)] Shutdown RemoteCommandsInvalidator.
[13828:13828:1204/200714.943008:INFO:remote_commands_invalidator.cc(68)] Stopping RemoteCommandsInvalidator.
[13828:13828:1204/200714.943021:INFO:remote_commands_invalidator.cc(167)] Unregister RemoteCommandsInvalidator.
[13828:13828:1204/200714.967654:WARNING:pref_notifier_impl.cc(23)] Pref observer found at shutdown.
[13828:13828:1204/200714.975105:WARNING:pref_notifier_impl.cc(23)] Pref observer found at shutdown.
[  FAILED  ] WebBluetoothTest.NavigateWithChooserCrossOrigin, where TypeParam =  and GetParam() =  (1197 ms)

The frame swap issues should have been solved in  issue 898645 . Is there corresponding flakiness in WebUsbTest.NavigateWithChooserCrossOrigin?

#c03: thanks for the info. Indeed the failure of WebUsbTest.NavigateWithChooserCrossOrigin (https://chromium-swarm.appspot.com/task?id=40c06a6dd989e010&refresh=10&show_raw=1) looks quite similar, though I am not particularly sure the correspondence with this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cee86b1312c2a9df3f74eee193ac522c6282b729

commit cee86b1312c2a9df3f74eee193ac522c6282b729
Author: Jun Mukai <mukai@chromium.org>
Date: Mon Dec 10 20:01:37 2018

Disable WebBluetoothTest.NavigateWithChooserCrossOrigin for SPM

This test case is flaky on single_process_mash, failing multiple
times in several days.

Bug:  912213 
Test: None
Change-Id: If0daa1cd506c192b32bd363ffc2f1e4b711eb770
Reviewed-on: https://chromium-review.googlesource.com/c/1367911
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615210}
[modify] https://crrev.com/cee86b1312c2a9df3f74eee193ac522c6282b729/testing/buildbot/filters/chromeos.single_process_mash.browser_tests.filter

Looks like it is flaky on regular brower_tests too

https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-rel/17151

Investigating. I couldn't reproduce it locally in 100 runs, trying 1000 and grabbing coffee.

Sometimes, it is easier to repro when making your dev box slower to simulate load on bots. I use "stress -c 32" to run 32 threads doing sqrt to make cpu busy and sometimes try "-c 64" or even "-c 128". 

You can install stress via "sudo apt-get install stress".

I couldn't get this to fail even using 'stress'. Also, using findit, I found 2 instances on single_process_mash_browser_tests and one on vis_browser_tests.

I plan to re-enable this test and keep an eye on things.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d12acb497b19557c29f141f823943f15438f5c6b

commit d12acb497b19557c29f141f823943f15438f5c6b
Author: Steven Bennetts <stevenjb@chromium.org>
Date: Mon Dec 17 18:14:37 2018

Remove some flaky SingleProcessMash tests from filters

This restores to browser_tests:
WebBluetoothTest.NavigateWithChooserCrossOrigin

This restores to unit_tests:
AdaptiveScreenBrightnessManagerTest.SingleBrowser

Bug:  912213 ,  913577 
Change-Id: I70896b461987a1a8d26623e464fbf64adad433b6
Reviewed-on: https://chromium-review.googlesource.com/c/1378824
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617161}
[modify] https://crrev.com/d12acb497b19557c29f141f823943f15438f5c6b/testing/buildbot/filters/chromeos.single_process_mash.browser_tests.filter
[modify] https://crrev.com/d12acb497b19557c29f141f823943f15438f5c6b/testing/buildbot/filters/chromeos.single_process_mash.unit_tests.filter

Marking Fixed untill proven otherwise.