Example builds:

https://ci.chromium.org/buildbot/chromium.clang/ToTWin%28dll%29/1864
https://ci.chromium.org/buildbot/chromium.clang/ToTWin64%28dll%29/1769

This looks too consistent to be flake.
Doesn't show on non-component builds.
Haven't verified whether it also reproduces with pinned clang yet.

From the latter build:

[ RUN      ] BrowserMainLoopTest.CreateThreadsInSingleProcess
[5116:6476:1107/020752.121:20935224:FATAL:thread_restrictions.cc(34)] Check failed: !g_blocking_disallowed.Get().Get(). Function marked as blocking was called from a scope that disallows blocking! If this task is running inside the TaskScheduler, it needs to have MayBlock() in its TaskTraits. Otherwise, consider making this blocking work asynchronous or, as a last resort, you may use ScopedAllowBlocking (see its documentation for best practices).
Backtrace:
	base::debug::StackTrace::StackTrace [0x000007FEF0BE9044+36] (C:\b\c\b\ToTWin64_dll_\src\base\debug\stack_trace_win.cc:289)
	logging::LogMessage::~LogMessage [0x000007FEF0C1647C+92] (C:\b\c\b\ToTWin64_dll_\src\base\logging.cc:593)
	base::AssertBlockingAllowedDeprecated [0x000007FEF0D03CEB+123] (C:\b\c\b\ToTWin64_dll_\src\base\threading\thread_restrictions.cc:46)
	net::GetNetworkList [0x000007FEEF7FA04D+66] (C:\b\c\b\ToTWin64_dll_\src\net\base\network_interfaces_win.cc:217)
	net::NetworkChangeNotifier::ConnectionTypeFromInterfaces [0x000007FEEF7E976D+49] (C:\b\c\b\ToTWin64_dll_\src\net\base\network_change_notifier.cc:471)
	net::NetworkChangeNotifierWin::RecomputeCurrentConnectionType [0x000007FEEF7F59AD+647] (C:\b\c\b\ToTWin64_dll_\src\net\base\network_change_notifier_win.cc:203)
	net::NetworkChangeNotifierWin::NetworkChangeNotifierWin [0x000007FEEF7F5484+156] (C:\b\c\b\ToTWin64_dll_\src\net\base\network_change_notifier_win.cc:62)
	net::NetworkChangeNotifier::Create [0x000007FEEF7E9262+50] (C:\b\c\b\ToTWin64_dll_\src\net\base\network_change_notifier.cc:207)
	network::NetworkService::NetworkService [0x000007FEE6D5E586+862] (C:\b\c\b\ToTWin64_dll_\src\services\network\network_service.cc:181)
	content::`anonymous namespace'::CreateNetworkServiceOnIO [0x000007FEE3A947F8+165] (C:\b\c\b\ToTWin64_dll_\src\content\browser\network_service_instance.cc:49)
	base::internal::Invoker<base::internal::BindState<void (*)(mojo::InterfaceRequest<network::mojom::NetworkService>),mojo::InterfaceRequest<network::mojom::NetworkService> >,void ()>::RunOnce [0x000007FEE3A95832+62] (C:\b\c\b\ToTWin64_dll_\src\base\bind_internal.h:658)
	base::debug::TaskAnnotator::RunTask [0x000007FEF0BEAB1C+364] (C:\b\c\b\ToTWin64_dll_\src\base\debug\task_annotator.cc:99)
	base::MessageLoopImpl::RunTask [0x000007FEF0C2EF7F+287] (C:\b\c\b\ToTWin64_dll_\src\base\message_loop\message_loop_impl.cc:406)
	base::MessageLoopImpl::DoWork [0x000007FEF0C2F5B5+389] (C:\b\c\b\ToTWin64_dll_\src\base\message_loop\message_loop_impl.cc:499)
	base::MessagePumpForIO::DoRunLoop [0x000007FEF0C33AE5+165] (C:\b\c\b\ToTWin64_dll_\src\base\message_loop\message_pump_win.cc:512)
	base::MessagePumpWin::Run [0x000007FEF0C31F3E+78] (C:\b\c\b\ToTWin64_dll_\src\base\message_loop\message_pump_win.cc:54)
	base::MessageLoopImpl::Run [0x000007FEF0C2EBEB+139] (C:\b\c\b\ToTWin64_dll_\src\base\message_loop\message_loop_impl.cc:358)
	base::RunLoop::Run [0x000007FEF0C7EA49+249] (C:\b\c\b\ToTWin64_dll_\src\base\run_loop.cc:105)
	base::Thread::Run [0x000007FEF0CFE338+200] (C:\b\c\b\ToTWin64_dll_\src\base\threading\thread.cc:257)
	content::BrowserProcessSubThread::IOThreadRun [0x000007FEE373BCB3+37] (C:\b\c\b\ToTWin64_dll_\src\content\browser\browser_process_sub_thread.cc:175)
	content::BrowserProcessSubThread::Run [0x000007FEE373BC2F+215] (C:\b\c\b\ToTWin64_dll_\src\content\browser\browser_process_sub_thread.cc:132)
	base::Thread::ThreadMain [0x000007FEF0CFE94E+766] (C:\b\c\b\ToTWin64_dll_\src\base\threading\thread.cc:356)
	base::`anonymous namespace'::ThreadFunc [0x000007FEF0CF861C+204] (C:\b\c\b\ToTWin64_dll_\src\base\threading\platform_thread_win.cc:103)
	BaseThreadInitThunk [0x00000000770459ED+13]
	RtlUserThreadStart [0x000000007717B371+33]