https://chromium-review.googlesource.com/c/chromium/src/+/1290571 added an exclusion for the test

 SelectFileDialogExtensionBrowserTest/
      SelectFileVirtualKeyboard_TabletMode

which hangs on the single process mash bots.  Trace is:

[ RUN      ] SelectFileDialogExtensionBrowserTest.SelectFileVirtualKeyboard_TabletMode
[26311:26311:1018/224045.903449:WARNING:user_policy_manager_factory_chromeos.cc(208)] No policy loaded for known non-enterprise user
[26311:26311:1018/224045.932237:ERROR:external_web_apps.cc(176)] Determining directory
[26311:26311:1018/224045.932285:ERROR:external_web_apps.cc(207)] Scanning /usr/share/chromium/extensions/web_apps
[26311:26311:1018/224045.935886:INFO:easy_unlock_service_regular.cc(163)] DeviceSyncClient is not ready yet, delaying UseLoadedRemoteDevices().
[26311:26311:1018/224046.031258:ERROR:external_web_apps.cc(176)] Determining directory
[26311:26311:1018/224046.031296:ERROR:external_web_apps.cc(207)] Scanning
[26311:26311:1018/224046.056745:WARNING:wallpaper_controller_client.cc(358)] Cannot get wallpaper files id in RemovePolicyWallpaper. This should never happen under normal circumstances.
[26311:26311:1018/224046.233741:INFO:secure_channel_service.cc(25)] SecureChannelService::OnStart()
[26311:26311:1018/224046.233769:INFO:secure_channel_initializer.cc(64)] SecureChannelInitializer::SecureChannelInitializer(): Fetching Bluetooth adapter. All requests received before the adapter is fetched will be queued.
[26311:26311:1018/224046.233811:INFO:secure_channel_service.cc(38)] SecureChannelService::OnBindInterface() for interface chromeos.secure_channel.mojom.SecureChannel.
[26311:26311:1018/224046.233919:INFO:device_sync_service.cc(30)] DeviceSyncService::OnStart()
[26311:26311:1018/224046.233960:INFO:device_sync_service.cc(46)] DeviceSyncService::OnBindInterface() from interface chromeos.device_sync.mojom.DeviceSync.
[26311:26311:1018/224046.234078:INFO:multidevice_setup_service.cc(62)] MultiDeviceSetupService::OnStart()
[26311:26311:1018/224046.234330:INFO:multidevice_setup_service.cc(75)] MultiDeviceSetupService::OnBindInterface() from interface chromeos.multidevice_setup.mojom.MultiDeviceSetup.
[26311:26311:1018/224046.247796:INFO:secure_channel_initializer.cc(119)] SecureChannelInitializer::OnBluetoothAdapterReceived(): Bluetooth adapter has been fetched. Passing all queued requests to the service.
[26311:26311:1018/224046.251740:ERROR:gpu_interface_provider.cc(87)] Not implemented reached in virtual void content::GpuInterfaceProvider::RegisterOzoneGpuInterfaces(service_manager::BinderRegistry *)
[26311:26311:1018/224046.301697:ERROR:layer_tree_host_impl.cc(3113)] Forcing zero-copy tile initialization as worker context is missing
[26311:26311:1018/224046.301964:INFO:multidevice_setup_service.cc(75)] MultiDeviceSetupService::OnBindInterface() from interface chromeos.multidevice_setup.mojom.MultiDeviceSetup.
[26311:26311:1018/224046.328002:ERROR:remote_text_input_client.cc(158)] Not implemented reached in virtual void RemoteTextInputClient::OnInputMethodChanged()
[26311:26311:1018/224046.328035:ERROR:remote_text_input_client.cc(115)] Not implemented reached in virtual ui::TextInputClient::FocusReason RemoteTextInputClient::GetFocusReason() const
[26311:26311:1018/224046.328042:ERROR:remote_text_input_client.cc(200)] Not implemented reached in virtual bool RemoteTextInputClient::ShouldDoLearning()
[26311:26311:1018/224046.328062:ERROR:remote_text_input_client.cc(109)] Not implemented reached in virtual bool RemoteTextInputClient::HasCompositionText() const
[26311:26311:1018/224046.328067:ERROR:remote_text_input_client.cc(121)] Not implemented reached in virtual bool RemoteTextInputClient::GetTextRange(gfx::Range *) const
[26311:26311:1018/224046.328076:ERROR:remote_text_input_client.cc(176)] Not implemented reached in virtual void RemoteTextInputClient::EnsureCaretNotInRect(const gfx::Rect &)
[26311:26311:1018/224046.340643:ERROR:layer_tree_host_impl.cc(3113)] Forcing zero-copy tile initialization as worker context is missing
[26311:26311:1018/224046.805114:ERROR:render_widget_host_view_aura.cc(1285)] Not implemented reached in virtual base::i18n::TextDirection content::RenderWidgetHostViewAura::GetTextDirection() const
[26311:26311:1018/224046.805243:INFO:select_file_dialog_extension_browsertest.cc(227)] Waiting for JavaScript ready message.
[26311:26311:1018/224046.835422:WARNING:shelf_button.cc(362)] An icon of size 32x32is being scaled up and will look blurry.
[26311:26311:1018/224046.866669:WARNING:shelf_button.cc(362)] An icon of size 32x32is being scaled up and will look blurry.
[26311:26311:1018/224046.866934:WARNING:shelf_button.cc(362)] An icon of size 32x32is being scaled up and will look blurry.
[26311:26311:1018/224046.867036:WARNING:shelf_button.cc(362)] An icon of size 32x32is being scaled up and will look blurry.
[26311:26311:1018/224046.894122:ERROR:layer_tree_host_impl.cc(3113)] Forcing zero-copy tile initialization as worker context is missing
[26311:26311:1018/224047.180772:INFO:CONSOLE(0)] "HTML Imports is deprecated and will be removed in M73, around March 2019. Please use ES modules instead. See https://www.chromestatus.com/features/5144752345317376 for more details.", source:  (0)
[26311:26311:1018/224047.180855:INFO:CONSOLE(0)] ":unresolved pseudo selector is deprecated and will be removed in M73, around March 2019. Please use :not(:defined) instead. See https://www.chromestatus.com/features/4642138092470272 for more details.", source:  (0)
[26311:26311:1018/224047.302033:INFO:CONSOLE(5489)] "Requesting volume list.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5489)
[26311:26311:1018/224047.319016:INFO:CONSOLE(5491)] "Volume list fetched with: 1 items.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5491)
[26311:26311:1018/224047.319095:INFO:CONSOLE(5501)] "Initializing volume: downloads:Downloads", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5501)
[26311:26311:1018/224047.319141:INFO:CONSOLE(5955)] "Requesting file system: downloads downloads:Downloads", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5955)
[26311:26311:1018/224047.319194:INFO:CONSOLE(720)] "Cache database creating or upgrading.", source: chrome-extension://pmfjbimdmchhbnneeidfognadeopoehp/background_scripts.js (720)
[26311:26311:1018/224047.347241:INFO:CONSOLE(6005)] "File system obtained: downloads:Downloads", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (6005)
[26311:26311:1018/224047.359379:INFO:CONSOLE(5505)] "Initialized volume: downloads:Downloads", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5505)
[26311:26311:1018/224047.359456:INFO:CONSOLE(5509)] "Initialized all volumes.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5509)
[26311:26311:1018/224047.374340:INFO:CONSOLE(442)] "document.registerElement is deprecated and will be removed in M73, around March 2019. Please use window.customElements.define instead. See https://www.chromestatus.com/features/4642138092470272 for more details.", source: chrome://resources/polymer/v1_0/polymer/polymer-micro-extracted.js (442)
[26311:26311:1018/224048.020624:INFO:CONSOLE(2083)] "Element.createShadowRoot is deprecated and will be removed in M73, around March 2019. Please use Element.attachShadow instead. See https://www.chromestatus.com/features/4507242028072960 for more details.", source: chrome://resources/polymer/v1_0/polymer/polymer-mini-extracted.js (2083)
[26311:26311:1018/224048.466472:INFO:select_file_dialog_extension_browsertest.cc(231)] Waiting for JavaScript dialog-ready message.
[26311:26311:1018/224048.479842:INFO:CONSOLE(75)] "[unhandled-error]: Unknown error.
Error: original stack
    at console.error.args [as error] (chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/foreground/js/main_scripts.js:72:19)
    at chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/foreground/js/main_scripts.js:51095:19", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/foreground/js/main_scripts.js (75)
[26311:26311:1018/224048.537276:INFO:CONSOLE(75)] "[unhandled-error]: Error retrieving Web Store access token.
Error: original stack
    at console.error.args [as error] (chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/foreground/js/main_scripts.js:72:19)
    at window.addEventListener (chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/foreground/js/main_scripts.js:55:11)", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/foreground/js/main_scripts.js (75)
[26311:26311:1018/224048.537438:INFO:CONSOLE(18842)] "Uncaught (in promise) Error retrieving Web Store access token.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/foreground/js/main_scripts.js (18842)

^^ those errors are normal, so no worries about them.

[26311:26311:1018/224048.819435:INFO:select_file_dialog_extension_browsertest.cc(197)] ClickElement #filename-input-textbox (441,584)
BrowserTestBase received signal: Terminated. Backtrace:
#0 0x00000538cdaf base::debug::StackTrace::StackTrace()
#1 0x0000059b9485 content::(anonymous namespace)::DumpStackTraceSignalHandler()
#2 0x7f34bd339cb0 
#3 0x7f34bd4016d3 epoll_wait
#4 0x0000060d9ced epoll_dispatch
#5 0x0000060d77dc event_base_loop
#6 0x0000053aaeeb base::MessagePumpLibevent::Run()
#7 0x0000052fafa4 base::MessageLoop::Run()
#8 0x0000053237c9 base::RunLoop::Run()
#9 0x0000060d1291 keyboard::WaitUntilShown()
#10 0x000001a6c2c3 SelectFileDialogExtensionBrowserTest_SelectFileVirtualKeyboard_TabletMode_Test::RunTestOnMainThread()
#11 0x0000059b8912 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#12 0x00000546af38 ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#13 0x000005469c85 ChromeBrowserMainParts::PreMainMessageLoopRun()
#14 0x0000021d3f17 chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()
#15 0x00000365451a content::BrowserMainLoop::PreMainMessageLoopRun()
#16 0x000003b0dd95 content::StartupTaskRunner::RunAllTasksNow()
#17 0x00000365307a content::BrowserMainLoop::CreateStartupTasks()
#18 0x000003656b30 content::BrowserMainRunnerImpl::Initialize()
#19 0x000003650c12 content::BrowserMain()
#20 0x0000050759de content::ContentMainRunnerImpl::Run()
#21 0x0000075b1135 service_manager::Main()
#22 0x000005073d24 content::ContentMain()
#23 0x0000059b84db content::BrowserTestBase::SetUp()
#24 0x00000540a96b InProcessBrowserTest::SetUp()
#25 0x000001a6d7c6 SelectFileDialogExtensionBrowserTest::SetUp()
#26 0x0000026e1c8d testing::Test::Run()
#27 0x0000026e28b0 testing::TestInfo::Run()
#28 0x0000026e2dc7 testing::TestCase::Run()
#29 0x0000026eeb97 testing::internal::UnitTestImpl::RunAllTests()
#30 0x0000026ee70d testing::UnitTest::Run()
#31 0x000005422841 base::TestSuite::Run()
#32 0x0000052d501d ChromeTestSuiteRunner::RunTestSuite()
#33 0x0000059dcb5c content::LaunchTests()
#34 0x0000052d54e3 LaunchChromeTests()
#35 0x0000052d4fae main
#36 0x7f34bd324f45 __libc_start_main
#37 0x000000ad84ea _start

TIME-OUT