First failure is https://build.chromium.org/p/chromium.webrtc/builders/Win8%20Tester/builds/28938

[9468:10628:0930/181004:FATAL:json_pref_store.cc(385)] Check failed: !has_pending_write_callbacks_. 
Backtrace:
	base::debug::StackTrace::StackTrace [0x02593CF7+23]
	logging::LogMessage::~LogMessage [0x0254B061+49]
	JsonPrefStore::RegisterOnNextWriteSynchronousCallbacks [0x03058FDF+154]
	JsonPrefStore::SerializeData [0x03059601+179]
	base::ImportantFileWriter::DoScheduledWrite [0x025D4B15+149]
	JsonPrefStore::CommitPendingWrite [0x0305803E+120]
	JsonPrefStore::~JsonPrefStore [0x03057DF8+21]
	scoped_refptr<extensions::WebRequestConditionAttribute const >::Release [0x050491C1+23]
	SegregatedPrefStore::~SegregatedPrefStore [0x03D49463+95]
	scoped_refptr<extensions::WebRequestConditionAttribute const >::Release [0x050491C1+23]
	PrefService::~PrefService [0x03053ED9+141]
	syncable_prefs::PrefServiceSyncable::~PrefServiceSyncable [0x03A78703+93]
	syncable_prefs::PrefServiceSyncable::`vector deleting destructor' [0x03A78714+11]
	ProfileImpl::~ProfileImpl [0x0264706B+271]
	ProfileDestroyer::DestroyProfileWhenAppropriate [0x0270A381+484]
	ProfileManager::ProfileInfo::~ProfileInfo [0x025F4D62+14]
	std::_Tree<std::_Tmap_traits<base::FilePath,linked_ptr<ProfileManager::ProfileInfo>,std::less<base::FilePath>,std::allocator<std::pair<base::FilePath const ,linked_ptr<ProfileManager::ProfileInfo> > >,0> >::_Erase [0x02544889+41]
	std::_Tree<std::_Tmap_traits<base::FilePath,linked_ptr<ProfileManager::ProfileInfo>,std::less<base::FilePath>,std::allocator<std::pair<base::FilePath const ,linked_ptr<ProfileManager::ProfileInfo> > >,0> >::clear [0x025448B2+13]
	std::_Tree<std::_Tmap_traits<base::FilePath,linked_ptr<ProfileManager::ProfileInfo>,std::less<base::FilePath>,std::allocator<std::pair<base::FilePath const ,linked_ptr<ProfileManager::ProfileInfo> > >,0> >::~_Tree<std::_Tmap_traits<base::FilePath,linked_p [0x025F4D0E+21]
	ProfileManager::~ProfileManager [0x025F4DAF+54]
	BrowserProcessImpl::StartTearDown [0x027C70E9+458]
	ChromeBrowserMainParts::PostMainMessageLoopRun [0x028233B0+304]
	content::BrowserMainLoop::ShutdownThreadsAndCleanUp [0x01ED327A+350]
	content::BrowserMainRunnerImpl::Shutdown [0x01ED4498+632]
	content::BrowserMain [0x01ECFE41+139]
	content::RunNamedProcessTypeMain [0x02536FAB+206]
	content::ContentMainRunnerImpl::Run [0x02536EAC+274]
	content::ContentMain [0x02536275+35]
	content::BrowserTestBase::SetUp [0x02963565+964]
	InProcessBrowserTest::SetUp [0x025E45D7+268]
	testing::internal::HandleExceptionsInMethodIfSupported<testing::Test,void> [0x02DE7AB9+32]
	testing::Test::Run [0x02DEEAA3+51]
	testing::TestCase::Run [0x02DEEB79+133]
	testing::internal::UnitTestImpl::RunAllTests [0x02DEEEF8+433]
	testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl,bool> [0x02DE7AFD+32]
	testing::UnitTest::Run [0x02DEED22+133]
	base::TestSuite::Run [0x025ED110+95]
	ChromeTestSuiteRunner::RunTestSuite [0x04C356CE+40]
	content::LaunchTests [0x0295D485+585]
	LaunchChromeTests [0x04C356A1+49]
	main [0x04C354BA+63]
	__scrt_common_main_seh [0x04BFAD1F+249] (f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:253)
	BaseThreadInitThunk [0x77527C04+36]
	RtlInitializeExceptionChain [0x777DAB8F+143]
	RtlInitializeExceptionChain [0x777DAB5A+90]