https://codereview.chromium.org/2630773002 is a culprit.

[ RUN      ] ContinueWhereILeftOffTest.Post
Xlib:  extension "RANDR" missing on display ":99".
Xlib:  extension "RANDR" missing on display ":99".
[17878:17878:0316/214010.466103:WARNING:audio_manager.cc(295)] Multiple instances of AudioManager detected
[17878:17878:0316/214010.466137:WARNING:audio_manager.cc(254)] Multiple instances of AudioManager detected
[17878:17878:0316/214010.572475:WARNING:password_store_factory.cc(247)] 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.
[17878:17878:0316/214010.664245:FATAL:desktop_window_tree_host_x11.cc(836)] Check failed: !is_active || window_mapped_in_server_.
#0 0x000002e5fe57 base::debug::StackTrace::StackTrace()
#1 0x000002e7887b logging::LogMessage::~LogMessage()
#2 0x000003de5765 views::DesktopWindowTreeHostX11::IsActive()
#3 0x000003de291f views::DesktopWindowTreeHostX11::AfterActivationStateChanged()
#4 0x000003de5601 views::DesktopWindowTreeHostX11::Activate()
#5 0x000003191781 (anonymous namespace)::SessionRestoreImpl::ProcessSessionWindowsAndNotify()
#6 0x0000031900ce SessionRestore::RestoreSession()
#7 0x0000053b76fa StartupBrowserCreatorImpl::RestoreOrCreateBrowser()
#8 0x0000053b6414 StartupBrowserCreatorImpl::ProcessLaunchUrlsUsingConsolidatedFlow()
#9 0x0000053b5cd1 StartupBrowserCreatorImpl::Launch()
#10 0x0000053b35f5 StartupBrowserCreator::LaunchBrowser()
#11 0x0000053b2eff StartupBrowserCreator::ProcessCmdLineImpl()
#12 0x0000053b2437 StartupBrowserCreator::Start()
#13 0x000003378884 ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#14 0x000003377811 ChromeBrowserMainParts::PreMainMessageLoopRun()
#15 0x000001ac0985 content::BrowserMainLoop::PreMainMessageLoopRun()
#16 0x000001eb1286 content::StartupTaskRunner::RunAllTasksNow()
#17 0x000001abe81b content::BrowserMainLoop::CreateStartupTasks()
#18 0x000001ac456e content::BrowserMainRunnerImpl::Initialize()
#19 0x000001abbc98 content::BrowserMain()
#20 0x000002e3b613 content::RunNamedProcessTypeMain()
#21 0x000002e3c326 content::ContentMainRunnerImpl::Run()
#22 0x000002e3a924 content::ContentMain()
#23 0x000003550b80 content::BrowserTestBase::SetUp()
#24 0x000002f36cf1 InProcessBrowserTest::SetUp()
#25 0x000003d2550e testing::Test::Run()
#26 0x000003d260d0 testing::TestInfo::Run()
#27 0x000003d26447 testing::TestCase::Run()
#28 0x000003d2d697 testing::internal::UnitTestImpl::RunAllTests()
#29 0x000003d2d317 testing::UnitTest::Run()
#30 0x000002f49e51 base::TestSuite::Run()
#31 0x000002e50368 ChromeTestSuiteRunner::RunTestSuite()
#32 0x00000358067c content::LaunchTests()
#33 0x000002e502e7 main
#34 0x7f6d9f4d7f45 __libc_start_main
#35 0x000000605531 <unknown>

[1:1:0100/000000.871744:ERROR:broker_posix.cc(41)] Invalid node channel message
[1:1:0100/000000.885035:ERROR:broker_posix.cc(41)] Invalid node channel message

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9b35aa2cb18ff78d6cfc843f0e9414ccb6c22150

commit 9b35aa2cb18ff78d6cfc843f0e9414ccb6c22150
Author: thomasanderson <thomasanderson@google.com>
Date: Fri Mar 17 19:45:27 2017

X11: Fix test flakes related to window focus

This CL removes code intended to fix "some tests depending on a window
being IsActive() immediately after an Activate()".  After
https://codereview.chromium.org/2630773002, all of these cases should
be using WidgetActivationWaiter, so this is unnecessary.  This was
causing DCHECK(!is_active || window_mapped_in_server_) to fail.

In addition, XSetInputFocus on an unmapped window will fail, so
supress the error in this case.  This is the same thing GTK does
https://github.com/GNOME/gtk/blob/7ade6e3f4668220f0fa1382944a87e63691ad9ad/gdk/x11/gdkwindow-x11.c#L1904

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_dbg_ng
BUG= 701722 , 701921 , 701706 , 701485 , 701484 , 700692 , 700683 , 700661 , 701886 
R=erg@chromium.org

Review-Url: https://codereview.chromium.org/2756183003
Cr-Commit-Position: refs/heads/master@{#457859}

[modify] https://crrev.com/9b35aa2cb18ff78d6cfc843f0e9414ccb6c22150/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc

Detected 3 new flakes for test/step "ContinueWhereILeftOffTest.Post". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyKQsSBUZsYWtlIh5Db250aW51ZVdoZXJlSUxlZnRPZmZUZXN0LlBvc3QM. This message was posted automatically by the chromium-try-flakes app. Since flakiness is ongoing, the issue was moved back into Sheriff Bug Queue (unless already there).

Isn't it already fixed?