I'm seeing an occasional crash in what looks like test teardowns for mash_browser_tests.

I haven't seen this locally, nor does it seem tied to a particular test case:

[0808/043235.161574:FATAL:interface_endpoint_client.cc(32)] Check failed: !is_valid. The callback passed to NativeDisplayDelegate::Configure() was never run.
#0 0x0000034bf97c base::debug::StackTrace::StackTrace()
#1 0x0000034d846c logging::LogMessage::~LogMessage()
#2 0x00000498e732 mojo::(anonymous namespace)::DCheckIfInvalid()
#3 0x000001da9ffb _ZN4base8internal9BindStateIMN7display5mojom48NativeDisplayDelegate_Configure_ProxyToResponderEFvbEJNS0_13PassedWrapperINSt3__110unique_ptrIS4_NS8_14default_deleteIS4_EEEEEEEED2Ev
#4 0x000001da9f46 _ZN4base8internal9BindStateIMN7display5mojom48NativeDisplayDelegate_Configure_ProxyToResponderEFvbEJNS0_13PassedWrapperINSt3__110unique_ptrIS4_NS8_14default_deleteIS4_EEEEEEEE7DestroyEPKNS0_13BindStateBaseE
#5 0x0000008ff407 _ZN4base8internal9BindStateINS0_18IgnoreResultHelperIPFbN7content13BrowserThread2IDERKN15tracked_objects8LocationENS_8CallbackIFvvELNS0_8CopyModeE0ELNS0_10RepeatModeE0EEEEEEJS5_S7_NSA_ISB_LSC_1ELSD_1EEEEE7DestroyEPKNS0_13BindStateBaseE
#6 0x0000006a1c77 _ZN4base8internal9BindStateINS_8CallbackIFvN6device5mojom17UsbTransferStatusE13scoped_refptrIN3net8IOBufferEEmELNS0_8CopyModeE1ELNS0_10RepeatModeE1EEEJS5_DniEE7DestroyEPKNS0_13BindStateBaseE
#7 0x0000008bed98 std::__1::__deque_base<>::clear()
#8 0x000004753cfe display::FakeDisplayDelegate::~FakeDisplayDelegate()
#9 0x000004753e0e display::FakeDisplayDelegate::~FakeDisplayDelegate()
#10 0x00000348b346 display::ScreenManagerForwarding::~ScreenManagerForwarding()
#11 0x00000348b3fe display::ScreenManagerForwarding::~ScreenManagerForwarding()
#12 0x000003480f73 ui::Service::~Service()
#13 0x00000348112e ui::Service::~Service()
#14 0x000003e31b2d service_manager::ServiceContext::~ServiceContext()
#15 0x0000034b0aaa (anonymous namespace)::StartEmbeddedService()
#16 0x000002193b0d _ZN4base8internal7InvokerINS0_9BindStateIPFvN4mojo16InterfaceRequestIN5blink5mojom19KeyboardLockServiceEEEEJEEES9_E3RunEPNS0_13BindStateBaseEOS8_
#17 0x000001e92256 service_manager::RunStandaloneService()
#18 0x0000034b08ee RunMashBrowserTests()
#19 0x0000034b0487 main
#20 0x7f2d9ca22f45 __libc_start_main
#21 0x0000005874b3 <unknown>
Received signal 6
0 0x0000034bf97c [0808/043235.304670:INFO:service_process_launcher.cc(191)] Launched child process pid=5640, instance=, name=ui, user_id=08274fd9-a1a4-401d-b900-948f2f70e8ef
[0808/043235.307330:INFO:service_process_launcher.cc(191)] Launched child process pid=5642, instance=, name=font_service, user_id=505C0EE9-3013-43C0-82B0-A84F50CF8D84
base::debug::StackTrace::StackTrace()
#1 0x0000034bf471 base::debug::(anonymous namespace)::StackDumpSignalHandler()
#2 0x7f2d9cdd9330 <unknown>
#3 0x7f2d9ca37c37 gsignal
#4 0x7f2d9ca3b028 abort
#5 0x0000034be175 base::debug::BreakDebugger()
#6 0x0000034d8912 logging::LogMessage::~LogMessage()
#7 0x00000498e732 mojo::(anonymous namespace)::DCheckIfInvalid()
#8 0x000001da9ffb _ZN4base8internal9BindStateIMN7display5mojom48NativeDisplayDelegate_Configure_ProxyToResponderEFvbEJNS0_13PassedWrapperINSt3__110unique_ptrIS4_NS8_14default_deleteIS4_EEEEEEEED2Ev
#9 0x000001da9f46 _ZN4base8internal9BindStateIMN7display5mojom48NativeDisplayDelegate_Configure_ProxyToResponderEFvbEJNS0_13PassedWrapperINSt3__110unique_ptrIS4_NS8_14default_deleteIS4_EEEEEEEE7DestroyEPKNS0_13BindStateBaseE
#10 0x0000008ff407 _ZN4base8internal9BindStateINS0_18IgnoreResultHelperIPFbN7content13BrowserThread2IDERKN15tracked_objects8LocationENS_8CallbackIFvvELNS0_8CopyModeE0ELNS0_10RepeatModeE0EEEEEEJS5_S7_NSA_ISB_LSC_1ELSD_1EEEEE7DestroyEPKNS0_13BindStateBaseE
#11 0x0000006a1c77 _ZN4base8internal9BindStateINS_8CallbackIFvN6device5mojom17UsbTransferStatusE13scoped_refptrIN3net8IOBufferEEmELNS0_8CopyModeE1ELNS0_10RepeatModeE1EEEJS5_DniEE7DestroyEPKNS0_13BindStateBaseE
#12 0x0000008bed98 std::__1::__deque_base<>::clear()
#13 0x000004753cfe display::FakeDisplayDelegate::~FakeDisplayDelegate()
#14 0x000004753e0e display::FakeDisplayDelegate::~FakeDisplayDelegate()
#15 0x00000348b346 display::ScreenManagerForwarding::~ScreenManagerForwarding()
#16 0x00000348b3fe [0808/043235.366322:INFO:service_process_launcher.cc(191)] Launched child process pid=5667, instance=, name=test_ime_driver, user_id=08274fd9-a1a4-401d-b900-948f2f70e8ef
display::ScreenManagerForwarding::~ScreenManagerForwarding()
#17 0x000003480f73 ui::Service::~Service()
#18 0x00000348112e ui::Service::~Service()
#19 0x000003e31b2d [0808/043235.384320:INFO:service_process_launcher.cc(191)] Launched child process pid=5685, instance=, name=test_ime_driver, user_id=e5606178-0e49-4d6e-a207-91372a7fdf2e
service_manager::ServiceContext::~ServiceContext()
#20 0x0000034b0aaa (anonymous namespace)::StartEmbeddedService()
#21 0x000002193b0d [0808/043235.386243:ERROR:shell_delegate_mus.cc(113)] Not implemented reached in virtual ash::GPUSupport *ash::ShellDelegateMus::CreateGPUSupport()
[0808/043235.386439:ERROR:shell_delegate_mus.cc(102)] Not implemented reached in virtual std::unique_ptr<PaletteDelegate> ash::ShellDelegateMus::CreatePaletteDelegate()
[0808/043235.386459:ERROR:shell_delegate_mus.cc(60)] Not implemented reached in virtual void ash::ShellDelegateMus::PreInit()
[0808/043235.389484:INFO:service_process_launcher.cc(191)] Launched child process pid=5687, instance=, name=content_packaged_services, user_id=7bb163b5-d8f0-486f-9813-1c1a3679a295
_ZN4base8internal7InvokerINS0_9BindStateIPFvN4mojo16InterfaceRequestIN5blink5mojom19KeyboardLockServiceEEEEJEEES9_E3RunEPNS0_13BindStateBaseEOS8_
#22 0x000001e92256 service_manager::RunStandaloneService()
#23 0x0000034b08ee [0808/043235.398212:ERROR:shell_delegate_mus.cc(113)] Not implemented reached in virtual ash::GPUSupport *ash::ShellDelegateMus::CreateGPUSupport()
[0808/043235.398407:ERROR:shell_delegate_mus.cc(102)] Not implemented reached in virtual std::unique_ptr<PaletteDelegate> ash::ShellDelegateMus::CreatePaletteDelegate()
[0808/043235.398426:ERROR:shell_delegate_mus.cc(60)] Not implemented reached in virtual void ash::ShellDelegateMus::PreInit()
RunMashBrowserTests()
#24 0x0000034b0487 [0808/043235.402251:INFO:service_process_launcher.cc(191)] Launched child process pid=5691, instance=, name=content_packaged_services, user_id=d6618946-103a-4c81-a248-58f557aab294
main
#25 0x7f2d9ca22f45 __libc_start_main
#26 0x0000005874b3 <unknown>
  r8: ffff8a252114fdf0  r9: ffff8a252114fde0 r10: 0000000000000008 r11: 0000000000000202
 r12: 00007fff201dbce0 r13: 00007fff201dbcd0 r14: 00007fff201dbcd8 r15: 00007fff201db830
  di: 00000000000014dc  si: 00000000000014dc  bp: 00007fff201db820  bx: 00007fff201db830
  dx: 0000000000000006  ax: 0000000000000000  cx: ffffffffffffffff  sp: 00007fff201db6e8
  ip: 00007f2d9ca37c37 efl: 0000000000000202 cgf: 0000000000000033 erf: 0000000000000000
 trp: 0000000000000000 msk: 0000000000000000 cr2: 0000000000000000
[end of stack trace]

It looks like the configuration of the fake display in the tests is failing.

Logs from an instance of this occurring: https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Fchromium.fyi%2FMojo_ChromiumOS%2F20779%2F%2B%2Frecipes%2Fsteps%2Fmash_browser_tests%2F0%2Fstdout