Can you please take a look at this and assign it to the right owner? Thank you!

Based on https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=network_service_browser_tests&tests=ExtensionWindowLastFocusedTest.NoDevtoolsAndAppWindows this appears to be timeout-flaky on windows specifically. I'm not convinced the culprit above is correct.

Given that it isn't flaking too hard as far as I can tell, downgrading priority.

At least some of these flakes don't seem timeouts at all? From the log in the initial description:
  [4712:2968:0930/184525.060:FATAL:interface_endpoint_client.cc(170)] Check failed: (sequence_checker_).CalledOnValidSequence().
Backtrace:
  	base::debug::StackTrace::StackTrace [0x7121C396+102]
  	base::debug::StackTrace::StackTrace [0x7121B36B+27]
  	logging::LogMessage::~LogMessage [0x7127FE94+148]
  	mojo::InterfaceEndpointClient::~InterfaceEndpointClient [0x6E442389+201]
  	mojo::InterfaceEndpointClient::`scalar deleting destructor' [0x6E444BD5+37]
  	std::default_delete<mojo::InterfaceEndpointClient>::operator() [0x6E4228A4+52]
  	std::unique_ptr<mojo::InterfaceEndpointClient,std::default_delete<mojo::InterfaceEndpointClient> >::reset [0x6E421E07+87]
  	mojo::internal::InterfacePtrStateBase::~InterfacePtrStateBase [0x6E450A57+39]
  	mojo::internal::InterfacePtrState<network::mojom::HostResolver>::~InterfacePtrState<network::mojom::HostResolver> [0x034806B6+38]
  	mojo::InterfacePtr<network::mojom::HostResolver>::~InterfacePtr<network::mojom::HostResolver> [0x034740CF+15]
  	TCPDeviceProvider::~TCPDeviceProvider [0x06445163+35]
  	TCPDeviceProvider::`scalar deleting destructor' [0x06445545+37]
  	base::RefCountedThreadSafe<AndroidDeviceManager::DeviceProvider,base::DefaultRefCountedThreadSafeTraits<AndroidDeviceManager::DeviceProvider> >::DeleteInternal<AndroidDeviceManager::DeviceProvider> [0x0038EA32+50]
  	base::DefaultRefCountedThreadSafeTraits<AndroidDeviceManager::DeviceProvider>::Destruct [0x0038E9F7+23]
  	base::RefCountedThreadSafe<AndroidDeviceManager::DeviceProvider,base::DefaultRefCountedThreadSafeTraits<AndroidDeviceManager::DeviceProvider> >::Release [0x0038E9D4+52]
  	scoped_refptr<AndroidDeviceManager::DeviceProvider>::Release [0x0038E995+21]
  	scoped_refptr<AndroidDeviceManager::DeviceProvider>::~scoped_refptr<AndroidDeviceManager::DeviceProvider> [0x0038D6D5+37]
  	std::_Tuple_val<scoped_refptr<AndroidDeviceManager::DeviceProvider> >::~_Tuple_val<scoped_refptr<AndroidDeviceManager::DeviceProvider> > [0x0640C81F+15]
  	std::tuple<scoped_refptr<AndroidDeviceManager::DeviceProvider>,std::basic_string<char,std::char_traits<char>,std::allocator<char> > >::~tuple<scoped_refptr<AndroidDeviceManager::DeviceProvider>,std::basic_string<char,std::char_traits<char>,std::allocator< [0x064188EB+27]
  	base::internal::BindState<void (__thiscall AndroidDeviceManager::DeviceProvider::*)(std::basic_string<char,std::char_traits<char>,std::allocator<char> > const &),scoped_refptr<AndroidDeviceManager::DeviceProvider>,std::basic_string<char,std::char_traits<c [0x064188BB+27]
  	base::internal::BindState<void (__thiscall AndroidDeviceManager::DeviceProvider::*)(std::basic_string<char,std::char_traits<char>,std::allocator<char> > const &),scoped_refptr<AndroidDeviceManager::DeviceProvider>,std::basic_string<char,std::char_traits<c [0x064187D3+35]
  	base::internal::BindStateBaseRefCountTraits::Destruct [0x711C19FA+26]
  	base::RefCountedThreadSafe<base::internal::BindStateBase,base::internal::BindStateBaseRefCountTraits>::Release [0x711BB140+48]
  	scoped_refptr<base::internal::BindStateBase>::Release [0x711C2252+18]

I.e. it seems TCPDeviceProvider might be getting destroyed on the wrong thread when part of some callback state being destroyed. 
+cduvall who did the refactoring that made it (more) important on which thread a TCPDeviceProvider is destroyed.

The test is still crashing with
[10088:2320:1011/022646.446:FATAL:interface_endpoint_client.cc(170)] Check failed: (sequence_checker_).CalledOnValidSequence().
  Backtrace:
  	base::debug::StackTrace::StackTrace [0x7002D346+102]
  	base::debug::StackTrace::StackTrace [0x7002C31B+27]
  	logging::LogMessage::~LogMessage [0x70090F04+148]
  	mojo::InterfaceEndpointClient::~InterfaceEndpointClient [0x6EE02379+201]
  	mojo::InterfaceEndpointClient::`scalar deleting destructor' [0x6EE04BC5+37]
  	std::default_delete<mojo::InterfaceEndpointClient>::operator() [0x6EDE28A4+52]
  	std::unique_ptr<mojo::InterfaceEndpointClient,std::default_delete<mojo::InterfaceEndpointClient> >::reset [0x6EDE1E07+87]
  	mojo::internal::InterfacePtrStateBase::~InterfacePtrStateBase [0x6EE10A47+39]
  	mojo::internal::InterfacePtrState<network::mojom::HostResolver>::~InterfacePtrState<network::mojom::HostResolver> [0x0411A976+38]
  	mojo::InterfacePtr<network::mojom::HostResolver>::~InterfacePtr<network::mojom::HostResolver> [0x0410E38F+15]
  	TCPDeviceProvider::~TCPDeviceProvider [0x071B2543+35]
  	TCPDeviceProvider::`scalar deleting destructor' [0x071B2925+37]
  	base::RefCountedThreadSafe<AndroidDeviceManager::DeviceProvider,base::DefaultRefCountedThreadSafeTraits<AndroidDeviceManager::DeviceProvider> >::DeleteInternal<AndroidDeviceManager::DeviceProvider> [0x00FB3ED2+50]
  	base::DefaultRefCountedThreadSafeTraits<AndroidDeviceManager::DeviceProvider>::Destruct [0x00FB3E97+23]
  	base::RefCountedThreadSafe<AndroidDeviceManager::DeviceProvider,base::DefaultRefCountedThreadSafeTraits<AndroidDeviceManager::DeviceProvider> >::Release [0x00FB3E74+52]
  	scoped_refptr<AndroidDeviceManager::DeviceProvider>::Release [0x00FB3E35+21]
  	scoped_refptr<AndroidDeviceManager::DeviceProvider>::~scoped_refptr<AndroidDeviceManager::DeviceProvider> [0x00FB2B75+37]
  	AndroidDeviceManager::DeviceDescriptor::~DeviceDescriptor [0x0716F513+35]
  	std::_Default_allocator_traits<std::allocator<AndroidDeviceManager::DeviceDescriptor> >::destroy<AndroidDeviceManager::DeviceDescriptor> [0x0717C4BC+28]
  	std::_Destroy_range1<std::allocator<AndroidDeviceManager::DeviceDescriptor> > [0x0717C47E+62]
  	std::_Destroy_range<std::allocator<AndroidDeviceManager::DeviceDescriptor> > [0x0717C428+72]
  	std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> >::_Destroy [0x0717C0B3+51]
  	std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> >::_Tidy [0x0717E386+86]
  	std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> >::~vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> > [0x0717E314+20]
  	std::default_delete<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> > >::operator() [0x0717E2E6+38]
  	std::unique_ptr<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> >,std::default_delete<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> > > > [0x0717338D+61]
  	std::_Tuple_val<std::unique_ptr<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> >,std::default_delete<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceD [0x0717B61F+15]
  	std::tuple<std::unique_ptr<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> >,std::default_delete<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescri [0x0717B5FF+15]
  	base::internal::BindState<base::RepeatingCallback<void __cdecl(std::unique_ptr<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> >,std::default_delete<std::vector<AndroidDeviceManager::DeviceDescript [0x0717B5CB+27]
  	base::internal::BindState<base::RepeatingCallback<void __cdecl(std::unique_ptr<std::vector<AndroidDeviceManager::DeviceDescriptor,std::allocator<AndroidDeviceManager::DeviceDescriptor> >,std::default_delete<std::vector<AndroidDeviceManager::DeviceDescript [0x0717B363+35]
  	base::internal::BindStateBaseRefCountTraits::Destruct [0x6FFD19FA+26]
  	base::RefCountedThreadSafe<base::internal::BindStateBase,base::internal::BindStateBaseRefCountTraits>::Release [0x6FFCB140+48]
  	scoped_refptr<base::internal::BindStateBase>::Release [0x6FFD2252+18]
  	scoped_refptr<base::internal::BindStateBase>::~scoped_refptr<base::internal::BindStateBase> [0x6FFD2115+37]
  	base::internal::CallbackBase::~CallbackBase [0x6FFD19CF+15]
  	base::OnceCallback<void __cdecl(void)>::~OnceCallback<void __cdecl(void)> [0x6FFCE91F+15]
  	base::PendingTask::~PendingTask [0x70168E8F+15]
  	base::MessageLoop::DoWork [0x700C67CE+446]
  	base::MessagePumpForUI::DoRunLoop [0x700D357E+62]
  	base::MessagePumpWin::Run [0x700D2507+119]
  	base::MessageLoop::Run [0x700C56B6+486]
  	base::RunLoop::Run [0x70196668+488]
  	content::WebContentsDestroyedWatcher::Wait [0x063DAC43+19]
  	extensions::PlatformAppBrowserTest::CloseAppWindow [0x021D7B9A+74]
  	extensions::ExtensionWindowLastFocusedTest_NoDevtoolsAndAppWindows_Test::RunTestOnMainThread [0x0249FB21+1537]
  	content::BrowserTestBase::ProxyRunTestOnMainThreadLoop [0x06335C3A+874]
  	??$Invoke@P8BrowserTestBase@content@@AEXXZPAV12@$$V@?$FunctorTraits@P8BrowserTestBase@content@@AEXXZX@internal@base@@SAXP8BrowserTestBase@content@@AEXXZ$$QAPAV34@@Z [0x0633830C+28]
  	base::internal::InvokeHelper<0,void>::MakeItSo<void (__thiscall content::BrowserTestBase::*const &)(void),content::BrowserTestBase *> [0x0633824F+79]
  	base::internal::Invoker<base::internal::BindState<void (__thiscall content::BrowserTestBase::*)(void),base::internal::UnretainedWrapper<content::BrowserTestBase> >,void __cdecl(void)>::RunImpl<void (__thiscall content::BrowserTestBase::*const &)(void),std [0x063381E5+85]
  	base::internal::Invoker<base::internal::BindState<void (__thiscall content::BrowserTestBase::*)(void),base::internal::UnretainedWrapper<content::BrowserTestBase> >,void __cdecl(void)>::Run [0x063380CF+63]
  	base::RepeatingCallback<void __cdecl(void)>::Run [0x00D2EB92+50]
  	ChromeBrowserMainParts::PreMainMessageLoopRunImpl [0x07DE9947+7447]
  	ChromeBrowserMainParts::PreMainMessageLoopRun [0x07DE7BAE+398]
  	content::BrowserMainLoop::PreMainMessageLoopRun [0x5EADE396+342]
  	??$Invoke@P8BrowserMainLoop@content@@AEHXZPAV12@$$V@?$FunctorTraits@P8BrowserMainLoop@content@@AEHXZX@internal@base@@SAHP8BrowserMainLoop@content@@AEHXZ$$QAPAV34@@Z [0x5EAE989C+28]
  	base::internal::InvokeHelper<0,int>::MakeItSo<int (__thiscall content::BrowserMainLoop::*const &)(void),content::BrowserMainLoop *> [0x5EAE97CF+79]
  	base::internal::Invoker<base::internal::BindState<int (__thiscall content::BrowserMainLoop::*)(void),base::internal::UnretainedWrapper<content::BrowserMainLoop> >,int __cdecl(void)>::RunImpl<int (__thiscall content::BrowserMainLoop::*const &)(void),std::t [0x5EAE9765+85]
  	base::internal::Invoker<base::internal::BindState<int (__thiscall content::BrowserMainLoop::*)(void),base::internal::UnretainedWrapper<content::BrowserMainLoop> >,int __cdecl(void)>::Run [0x5EAE960F+63]
  	base::RepeatingCallback<int __cdecl(void)>::Run [0x5F4FD312+50]
  	content::StartupTaskRunner::RunAllTasksNow [0x5FEB35F9+137]
  	content::BrowserMainLoop::CreateStartupTasks [0x5EADC23B+1483]
  	content::BrowserMainRunnerImpl::Initialize [0x5EAEFF94+1940]

Assigning to cduvall@ per #4.

This doesn't seem to be flaking much anymore and I can't repro locally. Closing this, feel free to reopen if there are more flakes.