I'm not sure what's different with mus_browser_tests, but it's been periodically failing on the try servers. I don't see failures on the main waterfall, nor do I know when it started failing.

Here's the stack from https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.linux%2Flinux_chromium_chromeos_rel_ng%2F509809%2F%2B%2Frecipes%2Fsteps%2Fmus_browser_tests__with_patch_%2F0%2Flogs%2FBrowserOnly__x2f_MemlogBrowserTest.EndToEnd__x2f_0%2F0 

#0 0x0000036f29ac base::debug::StackTrace::StackTrace()
#1 0x000003d00c28 content::(anonymous namespace)::DumpStackTraceSignalHandler()
#2 0x7fb8c0ffdcb0 <unknown>
#3 0x7fb8c43cc95b __libc_send
#4 0x000001ff23ab mojo::edk::PlatformChannelWrite()
#5 0x0000055c2512 profiling::MemlogSenderPipe::Send()
#6 0x0000055c2ba1 profiling::(anonymous namespace)::DoSend()
#7 0x0000055c2cb0 profiling::(anonymous namespace)::HookAlloc()
#8 0x000003795664 operator new()
#9 0x000004be38b9 IPC::Message::EnsureMessageAttachmentSet()
#10 0x000004bf9161 mojo::internal::UnmappedNativeStructSerializerImpl::SerializeMessageContents()
#11 0x000001f3a10d mojo::internal::Serializer<>::Serialize()
#12 0x000001f399a9 mojo::internal::ArraySerializer<>::SerializeElements()
#13 0x000001f3984f mojo::internal::Serializer<>::Serialize()
#14 0x000001f36dad mojo::internal::Serializer<>::Serialize()
#15 0x000001f34a3a viz::mojom::CompositorFrameSinkProxy::SubmitCompositorFrame()
#16 0x000001fce53d viz::ClientLayerTreeFrameSink::SubmitCompositorFrame()
#17 0x000004cb2e36 cc::LayerTreeHostImpl::DrawLayers()
#18 0x000004d497c8 cc::SingleThreadProxy::DoComposite()
#19 0x000004d4a480 cc::SingleThreadProxy::ScheduledActionDrawIfPossible()
#20 0x000004d59834 cc::Scheduler::DrawIfPossible()
#21 0x000004d56a1b cc::Scheduler::ProcessScheduledActions()
#22 0x000004d566e0 cc::Scheduler::OnBeginImplFrameDeadline()
#23 0x0000006b2677 _ZN4base8internal7InvokerINS0_9BindStateIMN12_GLOBAL__N_116SimpleHttpServer10ConnectionEFvvEJNS_7WeakPtrIS5_EEEEEFvvEE7RunOnceEPNS0_13BindStateBaseE
#24 0x0000006b2677 _ZN4base8internal7InvokerINS0_9BindStateIMN12_GLOBAL__N_116SimpleHttpServer10ConnectionEFvvEJNS_7WeakPtrIS5_EEEEEFvvEE7RunOnceEPNS0_13BindStateBaseE
#25 0x0000036f3148 base::debug::TaskAnnotator::RunTask()
#26 0x0000037a70dc base::internal::IncomingTaskQueue::RunTask()
#27 0x000003713453 base::MessageLoop::RunTask()
#28 0x000003713a5e base::MessageLoop::DoWork()
#29 0x000003716279 base::MessagePumpLibevent::Run()
#30 0x000003712f62 base::MessageLoop::Run()
#31 0x00000373cc4a base::RunLoop::Run()
#32 0x00000152ee6f (anonymous namespace)::MemlogBrowserTest_EndToEnd_Test::RunTestOnMainThread()
#33 0x000003d009a3 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#34 0x00000382529c ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#35 0x00000382412d ChromeBrowserMainParts::PreMainMessageLoopRun()
#36 0x0000016a4b82 chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()
#37 0x0000022172a1 content::BrowserMainLoop::PreMainMessageLoopRun()
#38 0x000002606d57 content::StartupTaskRunner::RunAllTasksNow()
#39 0x00000221562f content::BrowserMainLoop::CreateStartupTasks()
#40 0x000002219bc0 content::BrowserMainRunnerImpl::Initialize()
#41 0x000002212c37 content::BrowserMain()
#42 0x00000369a06c content::ContentMainRunnerImpl::Run()
#43 0x00000556854e service_manager::Main()
#44 0x000003698a44 content::ContentMain()
#45 0x000003d00663 content::BrowserTestBase::SetUp()
#46 0x0000037e3e76 InProcessBrowserTest::SetUp()
#47 0x0000019eaa81 testing::Test::Run()
#48 0x0000019eb570 testing::TestInfo::Run()
#49 0x0000019eba57 testing::TestCase::Run()
#50 0x0000019f1fe7 testing::internal::UnitTestImpl::RunAllTests()
#51 0x0000019f1c67 testing::UnitTest::Run()
#52 0x0000037f92f2 base::TestSuite::Run()
#53 0x0000036e5ac9 ChromeTestSuiteRunner::RunTestSuite()
#54 0x000003d322bf content::LaunchTests()
#55 0x0000036e5e91 LaunchChromeTests()
#56 0x0000036e36ed RunMashBrowserTests()
#57 0x0000036e3397 main
#58 0x7fb8c0fe8f45 __libc_start_main

I will disable the test on mus_browser_tests in the short term. https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_chromeos_rel_ng/builds/509787 is the first failure I see in the 200 runs on the trybot.