I'm seeing this in trying to get views_aura_mus_unittests working. Here's the trace:
[16054:16054:1108/111333:508423601371:FATAL:surface_manager.cc(48)] Check failed: frame_sink_source_map_.size() == 0u (1 vs. 0)
#0 0x7fa34c1565fe base::debug::StackTrace::StackTrace()
#1 0x7fa34c1be51c logging::LogMessage::~LogMessage()
#2 0x7fa349fd3407 cc::SurfaceManager::~SurfaceManager()
#3 0x7fa34131c227 ui::DisplayCompositor::~DisplayCompositor()
#4 0x7fa34131c269 ui::DisplayCompositor::~DisplayCompositor()
#5 0x7fa34141416f base::RefCounted<>::Release()
#6 0x7fa341414119 scoped_refptr<>::Release()
#7 0x7fa341412daa scoped_refptr<>::~scoped_refptr()
#8 0x7fa3414534ab ui::ws::WindowServer::~WindowServer()
#9 0x7fa341453ba9 ui::ws::WindowServer::~WindowServer()
#10 0x7fa34115163f std::default_delete<>::operator()()
#11 0x7fa34114947c std::unique_ptr<>::reset()
#12 0x7fa341146cb1 ui::Service::~Service()
#13 0x7fa341147019 ui::Service::~Service()
#14 0x7fa3411467ef std::default_delete<>::operator()()
#15 0x7fa34139e82c std::unique_ptr<>::reset()
#16 0x7fa34139e699 std::unique_ptr<>::~unique_ptr()
#17 0x7fa34139d82b service_manager::ServiceContext::~ServiceContext()
#18 0x7fa34139d879 service_manager::ServiceContext::~ServiceContext()
#19 0x7fa3411467ef std::default_delete<>::operator()()
#20 0x7fa3413a590c std::unique_ptr<>::reset()
#21 0x7fa3413a54e9 service_manager::ServiceRunner::Run()
#22 0x7fa3413a557c service_manager::ServiceRunner::Run()
#23 0x7fa341146370 ServiceMain
#24 0x0000007a67ee service_manager::RunServiceInNativeLibrary()
#25 0x00000079c34e service_manager::(anonymous namespace)::RunNativeLibrary()
#26 0x00000079c852 _ZN4base8internal13FunctorTraitsIPFvPvN4mojo16InterfaceRequestIN15service_manager5mojom7ServiceEEEEvE6InvokeIJRKS2_S8_EEEvSA_DpOT_
#27 0x00000079c7cd _ZN4base8internal12InvokeHelperILb0EvE8MakeItSoIRKPFvPvN4mojo16InterfaceRequestIN15service_manager5mojom7ServiceEEEEJRKS4_SA_EEEvOT_DpOT0_
#28 0x00000079c777 _ZN4base8internal7InvokerINS0_9BindStateIPFvPvN4mojo16InterfaceRequestIN15service_manager5mojom7ServiceEEEEJS3_EEEFvS9_EE7RunImplIRKSB_RKSt5tupl\
eIJS3_EEJLm0EEEEvOT_OT0_NS_13IndexSequenceIJXspT1_EEEEOS9_
#29 0x00000079c6ac _ZN4base8internal7InvokerINS0_9BindStateIPFvPvN4mojo16InterfaceRequestIN15service_manager5mojom7ServiceEEEEJS3_EEEFvS9_EE3RunEPNS0_13BindStateBa\
seEOS9_
#30 0x0000006c8b93 base::internal::RunMixin<>::Run()
#31 0x0000007a33fd service_manager::ChildProcessMainWithCallback()
#32 0x00000079c2ed service_manager::ChildProcessMain()
#33 0x000000499bf6 service_manager::(anonymous namespace)::RunChildProcess()
#34 0x000000499bab main
#35 0x7fa347e4bf45 __libc_start_main
#36 0x000000492e69 <unknown>

To repro build views_aura_mus_unittests and run WidgetObserverTest.ClosingOnHiddenParent. The test actually passes, it's just shutdown that crashes.