It looks that the problem is in DCHECK in network annotation. Error message

5676:6396:0316/032955.663:FATAL:network_traffic_annotation.h(226)] Check failed: is_valid().
Backtrace:
	base::debug::StackTrace::StackTrace [0x00007FF63E2BA6E4+36]
	logging::LogMessage::~LogMessage [0x00007FF63E211A93+99]
	net::MutableNetworkTrafficAnnotationTag::operator net::NetworkTrafficAnnotationTag [0x00007FF63C5468C2+98]
	net::InitSocketHandleForHttpRequest [0x00007FF63E5FE657+2167]
	net::InitSocketHandleForHttpRequest [0x00007FF63E5FDF6C+396]
	net::HttpStreamFactoryImpl::Job::DoInitConnectionImpl [0x00007FF63E5F6405+2885]
	net::HttpStreamFactoryImpl::Job::DoInitConnection [0x00007FF63E5F4752+34]
	net::HttpStreamFactoryImpl::Job::DoLoop [0x00007FF63E5F4060+640]
	net::HttpStreamFactoryImpl::Job::RunLoop [0x00007FF63E5F1EC0+96]
	net::HttpStreamFactoryImpl::Job::StartInternal [0x00007FF63E5F1C15+133]
	net::HttpStreamFactoryImpl::JobController::DoCreateJobs [0x00007FF63E5FC6A4+2932]
	net::HttpStreamFactoryImpl::JobController::DoLoop [0x00007FF63E5FB6CB+347]
	net::HttpStreamFactoryImpl::JobController::RunLoop [0x00007FF63E5F83B8+40]
	net::ProxyResolutionService::Request::QueryComplete [0x00007FF63E49B0D4+84]
	net::ProxyResolutionService::SetReady [0x00007FF63E49873B+235]
	net::ProxyResolutionService::InitializeUsingLastFetchedConfig [0x00007FF63E49899E+446]
	net::ProxyResolutionService::OnProxyConfigChanged [0x00007FF63E49A2F9+409]
	network::ProxyConfigServiceMojo::OnProxyConfigUpdated [0x00007FF63F744D4A+186]
	network::mojom::ProxyConfigClientStubDispatch::Accept [0x00007FF63C54E852+402]
	mojo::InterfaceEndpointClient::HandleValidatedMessage [0x00007FF63EDFC3C0+656]
	mojo::FilterChain::Accept [0x00007FF63EE018F7+151]
	mojo::InterfaceEndpointClient::HandleIncomingMessage [0x00007FF63EDFD37A+122]
	mojo::internal::MultiplexRouter::ProcessIncomingMessage [0x00007FF63EDF5649+729]
	mojo::internal::MultiplexRouter::Accept [0x00007FF63EDF515C+348]
	mojo::FilterChain::Accept [0x00007FF63EE018F7+151]
	mojo::Connector::ReadSingleMessage [0x00007FF63EDF9B48+440]
	mojo::Connector::ReadAllAvailableMessages [0x00007FF63EDFA390+128]
	mojo::Connector::OnHandleReadyInternal [0x00007FF63EDFA1FB+139]
	mojo::SimpleWatcher::OnHandleReady [0x00007FF63ED4A522+258]
	base::debug::TaskAnnotator::RunTask [0x00007FF63E2B6788+296]
	base::internal::IncomingTaskQueue::RunTask [0x00007FF63E2DDF4E+126]
	base::MessageLoop::RunTask [0x00007FF63E24E079+681]
	base::MessageLoop::DeferOrRunPendingTask [0x00007FF63E24E4C7+183]
	base::MessageLoop::DoWork [0x00007FF63E24E70E+542]
	base::MessagePumpForIO::DoRunLoop [0x00007FF63E294215+165]
	base::MessagePumpWin::Run [0x00007FF63E2932F8+104]
	base::MessageLoop::Run [0x00007FF63E24D8D9+201]
	base::RunLoop::Run [0x00007FF63E22296C+252]
	base::Thread::Run [0x00007FF63E269B98+200]
	content::BrowserThreadImpl::IOThreadRun [0x00007FF63CD23477+39]
	content::BrowserThreadImpl::Run [0x00007FF63CD2361E+302]
	base::Thread::ThreadMain [0x00007FF63E26A08A+730]
	base::PlatformThread::GetCurrentThreadPriority [0x00007FF63E24201C+572]
	BaseThreadInitThunk [0x00007FFAC5E18364+20]
	RtlUserThreadStart [0x00007FFAC83B5E91+33]

Detected 3 new flakes for test/step "SystemNetworkContext/NetworkContextConfigurationBrowserTest.ProxyConfig/0". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyVAsSBUZsYWtlIklTeXN0ZW1OZXR3b3JrQ29udGV4dC9OZXR3b3JrQ29udGV4dENvbmZpZ3VyYXRpb25Ccm93c2VyVGVzdC5Qcm94eUNvbmZpZy8wDA. This message was posted automatically by the chromium-try-flakes app. Since flakiness is ongoing, the issue was moved back into Sheriff Bug Queue (unless already there).

Detected 6 new flakes for test/step "SystemNetworkContext/NetworkContextConfigurationBrowserTest.ProxyConfig/0". To see the actual flakes, please visit https://chromium-try-flakes.appspot.com/all_flake_occurrences?key=ahVzfmNocm9taXVtLXRyeS1mbGFrZXNyVAsSBUZsYWtlIklTeXN0ZW1OZXR3b3JrQ29udGV4dC9OZXR3b3JrQ29udGV4dENvbmZpZ3VyYXRpb25Ccm93c2VyVGVzdC5Qcm94eUNvbmZpZy8wDA. This message was posted automatically by the chromium-try-flakes app.

Since no action has been taken, I'm disabling the test.

Someone who is OOO generally can't take action. :(

Looks like the first failure was Match 16. rhalavati landed 15 Cls on the 15th and 16th, so going through to find the breakage seems like potentially a fair bit of effort.  I'll see if I can repro.  If not, I'll disable the failing DCHECK until he gets back.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/daf301c6167457e8f416a12d54a9eecf327e1e14

commit daf301c6167457e8f416a12d54a9eecf327e1e14
Author: Alexander Potapenko <glider@google.com>
Date: Wed Mar 21 17:39:40 2018

Disable flaky NetworkContextConfigurationBrowserTest.ProxyConfig

BUG= 823077 
TBR=rhalavati@chromium.org,eroman@chromium.org

Change-Id: I1aa9de378251746704f9c358de45f5ff1540397e
Reviewed-on: https://chromium-review.googlesource.com/973607
Reviewed-by: Alexander Potapenko <glider@chromium.org>
Commit-Queue: Alexander Potapenko <glider@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544761}
[modify] https://crrev.com/daf301c6167457e8f416a12d54a9eecf327e1e14/chrome/browser/net/network_context_configuration_browsertest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4196b8354cd1bf4aed168dfc0f21074893059303

commit 4196b8354cd1bf4aed168dfc0f21074893059303
Author: Ramin Halavati <rhalavati@chromium.org>
Date: Mon Apr 09 08:55:34 2018

Fix ProxyResolutionService annotation bug and reactivate tests.

Network traffic annotation in ProxyResolutionService was wrongly set in
cases where proxy info was fetched through completely synchronous
method, causing flaky test failures.
The bug is fixed and tests are re-enabled.

Bug:  822975 
Bug:  826744 
Bug:  823077 
Change-Id: Ic63301ff053fc9160b1a96372af1ee289342396c
Reviewed-on: https://chromium-review.googlesource.com/997656
Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549130}
[modify] https://crrev.com/4196b8354cd1bf4aed168dfc0f21074893059303/chrome/browser/net/network_context_configuration_browsertest.cc
[modify] https://crrev.com/4196b8354cd1bf4aed168dfc0f21074893059303/net/proxy_resolution/proxy_resolution_service.cc
[modify] https://crrev.com/4196b8354cd1bf4aed168dfc0f21074893059303/net/traffic_annotation/network_traffic_annotation.h

There seems to be no more errors. Marking it fixed.