After the change to "Migrate chrome/test/chromedriver/net/net_util.cc using SimpleURLLoader" (r601478), the following four unit tests started failing in debug builds:

    FetchUrlTest.ConnectionClose 
    FetchUrlTest.Http200 
    FetchUrlTest.HttpNon200 
    FetchUrlTest.NoServer

To repro, create a debug build configuration, and then build and run the tests with the following commands:

autoninja -C out/Default chromedriver_tests
out/Default/chromedriver_tests --gtest_filter=FetchUrlTest.*

Here is an example called stack at the point of failure:

[77076:77081:1106/184041.036301:640220973814:FATAL:interface_endpoint_client.cc(225)] Check failed: (sequence_checker_).CalledOnValidSequence(). 
#0 0x7f4190e7dc2d base::debug::StackTrace::StackTrace()
#1 0x7f4190b73cfa base::debug::StackTrace::StackTrace()
#2 0x7f4190be59db logging::LogMessage::~LogMessage()
#3 0x7f419102d89b mojo::InterfaceEndpointClient::Accept()
#4 0x7f418a65ac8c network::mojom::URLLoaderFactoryProxy::CreateLoaderAndStart()
#5 0x7f418f3ebc92 network::WeakWrapperSharedURLLoaderFactory::CreateLoaderAndStart()
#6 0x7f418f3d4ee5 network::(anonymous namespace)::SimpleURLLoaderImpl::StartRequest()
#7 0x7f418f3cf633 network::(anonymous namespace)::SimpleURLLoaderImpl::Start()
#8 0x7f418f3c9636 network::(anonymous namespace)::SimpleURLLoaderImpl::DownloadToStringOfUnboundedSizeUntilCrashAndDie()
#9 0x556e4e301471 (anonymous namespace)::SyncUrlFetcher::FetchOnIOThread()
#10 0x556e4e3017cd _ZN4base8internal13FunctorTraitsIMN12_GLOBAL__N_114SyncUrlFetcherEFvvEvE6InvokeIS5_PS3_JEEEvT_OT0_DpOT1_
#11 0x556e4e301714 _ZN4base8internal12InvokeHelperILb0EvE8MakeItSoIMN12_GLOBAL__N_114SyncUrlFetcherEFvvEJPS5_EEEvOT_DpOT0_
#12 0x556e4e3016c5 _ZN4base8internal7InvokerINS0_9BindStateIMN12_GLOBAL__N_114SyncUrlFetcherEFvvEJNS0_17UnretainedWrapperIS4_EEEEEFvvEE7RunImplIS6_NSt3__15tupleIJS8_EEEJLm0EEEEvOT_OT0_NSD_16integer_sequenceImJXspT1_EEEE
#13 0x556e4e301609 _ZN4base8internal7InvokerINS0_9BindStateIMN12_GLOBAL__N_114SyncUrlFetcherEFvvEJNS0_17UnretainedWrapperIS4_EEEEEFvvEE7RunOnceEPNS0_13BindStateBaseE
#14 0x7f4190b2341e _ZNO4base12OnceCallbackIFvvEE3RunEv
#15 0x7f4190b75342 base::debug::TaskAnnotator::RunTask()
#16 0x7f4190d9f7f2 base::internal::TaskTracker::RunOrSkipTask()
#17 0x7f4190eb80fc base::internal::TaskTrackerPosix::RunOrSkipTask()
#18 0x556e4e5aec81 base::test::ScopedTaskEnvironment::TestTaskTracker::RunOrSkipTask()
#19 0x7f4190d9d42c base::internal::TaskTracker::RunAndPopNextTask()
#20 0x7f4190d828c5 base::internal::SchedulerWorker::RunWorker()
#21 0x7f4190d8204c base::internal::SchedulerWorker::RunPooledWorker()
#22 0x7f4190d81f6f base::internal::SchedulerWorker::ThreadMain()
#23 0x7f4190eb960d base::(anonymous namespace)::ThreadFunc()
#24 0x7f418936d494 start_thread
#25 0x7f4186486a8f clone