Various unit_tests will fail on their first run, because they try to create a platform-specific NetworkChangeNotifier singleton, which some earlier test has already created, and leaked, e.g:

[ RUN      ] BackgroundSyncPermissionContextTest.TestSecureRequestingUrl
[23782:23782:0921/182138.238032:4001550231500:FATAL:network_change_notifier.cc(672)] Check failed: !g_network_change_notifier. 
#0 0x7f7d57e1415d base::debug::StackTrace::StackTrace()
#1 0x7f7d57b1b9ac base::debug::StackTrace::StackTrace()
#2 0x7f7d57b8b47a logging::LogMessage::~LogMessage()
#3 0x7f7d5c0f1f47 net::NetworkChangeNotifier::NetworkChangeNotifier()
#4 0x7f7d5c0f1214 net::(anonymous namespace)::MockNetworkChangeNotifier::MockNetworkChangeNotifier()
#5 0x7f7d5c0f11d2 net::NetworkChangeNotifier::CreateMock()
#6 0x0000076501b0 content::RenderViewHostTestHarness::SetUp()
#7 0x000004670a2e _ZN7testing8internal12InvokeHelperIvNSt3__15tupleIJEEEE12InvokeMethodIN10extensions27ExtensionDownloaderDelegateEMS8_FvvEEEvPT_T0_RKS4_
#8 0x00000560c392 testing::internal::HandleExceptionsInMethodIfSupported<>()
#9 0x0000055ed434 testing::Test::Run()
#10 0x0000055ede95 testing::TestInfo::Run()
#11 0x0000055ee94f testing::TestCase::Run()
#12 0x000005602148 testing::internal::UnitTestImpl::RunAllTests()
#13 0x00000561578e testing::internal::HandleSehExceptionsInMethodIfSupported<>()
#14 0x00000560da32 testing::internal::HandleExceptionsInMethodIfSupported<>()
#15 0x000005601da7 testing::UnitTest::Run()
#16 0x0000074fe8b1 RUN_ALL_TESTS()
#17 0x0000074fab0b base::TestSuite::Run()
#18 0x000007660520 content::UnitTestTestSuite::Run()
#19 0x000002416c6d _ZN4base8internal13FunctorTraitsIMNS_7RunLoopEFvvEvE6InvokeIS4_PS2_JEEEvT_OT0_DpOT1_
#20 0x000002416be4 _ZN4base8internal12InvokeHelperILb0EvE8MakeItSoIMNS_7RunLoopEFvvEJPS4_EEEvOT_DpOT0_
#21 0x000002416b95 _ZN4base8internal7InvokerINS0_9BindStateIMNS_7RunLoopEFvvEJNS0_17UnretainedWrapperIS3_EEEEEFvvEE7RunImplIS5_NSt3__15tupleIJS7_EEEJLm0EEEEvOT_OT0_NSC_16integer_sequenceImJXspT1_EEEE
#22 0x0000029eb29c _ZN4base8internal7InvokerINS0_9BindStateIM17ThreadWatcherTestFvvEJNS0_17UnretainedWrapperIS3_EEEEEFvvEE3RunEPNS0_13BindStateBaseE
#23 0x000005d2746e _ZNO4base12OnceCallbackIFivEE3RunEv
#24 0x000007503d85 base::(anonymous namespace)::LaunchUnitTestsInternal()
#25 0x000007503bd5 base::LaunchUnitTests()
#26 0x0000074dbed1 main
#27 0x7f7d305192b1 __libc_start_main
#28 0x00000240102a _start

The leaked instance of NetworkChangeNotifier appears to be created by the in-process NetworkService, which is itself a leaked singleton.

Firstly, we should be tearing-down the NetworkServiceImpl after each test, to ensure proper isolation.

Secondly, we remove the limitation of there being at-most-one NetworkChangeNotifier, and either allow multiple to be created, or have a leaked singleton.

Note that we currently retry 40-50 tests in this suite on every run, and each retry takes ~3s to execute, i.e. these and other issues requiring retries waste ~3 minutes of every run of unit_tests.