I've seen this across several browser_tests --mash, usually in ones that open a fullscreen window. For example:

ExtensionInstallUIBrowserTest.*
ImmersiveModeBrowserViewTest.*

[56492:56492:1117/132440.577515:FATAL:window_port_mus.cc(296)] Check failed: window_mus_type() == WindowMusType::TOP_LEVEL_IN_WM || window_mus_type() == WindowMusType::EMBED_IN_OWNER. 
#0 0x7f2030776f3c base::debug::StackTrace::StackTrace()
#1 0x7f203079d92c logging::LogMessage::~LogMessage()
#2 0x7f202b0f7ad1 aura::WindowPortMus::SetFrameSinkIdFromServer()
#3 0x7f202b10474e aura::WindowTreeClient::WmCreateTopLevelWindow()
#4 0x7f202b126999 ui::mojom::WindowManagerStubDispatch::Accept()
#5 0x7f202e0ea42b mojo::InterfaceEndpointClient::HandleValidatedMessage()
#6 0x7f202e0e9d26 mojo::FilterChain::Accept()
#7 0x7f202e0eb795 mojo::InterfaceEndpointClient::HandleIncomingMessage()
#8 0x7f202e0f59b2 mojo::internal::MultiplexRouter::ProcessIncomingMessage()
#9 0x7f202e0f4ee4 mojo::internal::MultiplexRouter::Accept()
#10 0x7f202e0e9d26 mojo::FilterChain::Accept()
#11 0x7f202e0e4dd5 mojo::Connector::ReadSingleMessage()
#12 0x7f202e0e58c1 mojo::Connector::ReadAllAvailableMessages()
#13 0x7f202e0e5769 mojo::Connector::OnHandleReadyInternal()
#14 0x7f202e0e5f67 mojo::SimpleWatcher::DiscardReadyState()
#15 0x7f202e0ae8b2 mojo::SimpleWatcher::OnHandleReady()
#16 0x7f202e0aedd1 _ZN4base8internal7InvokerINS0_9BindStateIMN4mojo13SimpleWatcherEFvijRKNS3_18HandleSignalsStateEEJNS_7WeakPtrIS4_EEijS5_EEEFvvEE7RunImplIRKS9_RKNSt3__15tupleIJSB_ijS5_EEEJLm0ELm1ELm2ELm3EEEEvOT_OT0_NSI_16integer_sequenceImJXspT1_EEEE
#17 0x7f2030777815 base::debug::TaskAnnotator::RunTask()
#18 0x7f20307a8ee9 base::internal::IncomingTaskQueue::RunTask()
#19 0x7f20307ac958 base::MessageLoop::RunTask()

erg mentioned in  issue 640365  that he's seen this when entering immersive mode with --mash, with the window type being LOCAL.

sky, feel free to reassign if you're not the right person for this.