Describe problem with existing test, or request for new test.

This happens on https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20ChromiumOS%20MSan%20Tests

What I am seeing is stack traces with postMessage being called on a null object:

[8411:8700:0827/235638.499417:WARNING:embedded_test_server.cc(238)] Request not handled. Returning 404: /favicon.ico
[8411:8411:0827/235638.645369:INFO:CONSOLE(44)] "sendResultToTest: ok - service worker registered", source: https://127.0.0.1:42246/result_queue.js (44)
[8411:8411:0827/235638.663022:INFO:CONSOLE(139)] "Uncaught (in promise) TypeError: Cannot read property 'postMessage' of null", source: https://127.0.0.1:42246/background_fetch/background_fetch.js (139)
BrowserTestBase received signal: Terminated. Backtrace:
    #0 0x000000ba9c31 in __interceptor_backtrace /b/swarming/w/ir/kitchen-workdir/src/third_party/llvm/compiler-rt/lib/msan/../sanitizer_common/sanitizer_common_interceptors.inc:4024:13
    #1 0x00001485b9cf in base::debug::StackTrace::StackTrace(unsigned long) ./../../base/debug/stack_trace_posix.cc:814:41
    #2 0x0000164db942 in content::(anonymous namespace)::DumpStackTraceSignalHandler(int) ./../../content/public/test/browser_test_base.cc:84:5
    #3 0x000000bd16a9 in SignalHandler(int) /b/swarming/w/ir/kitchen-workdir/src/third_party/llvm/compiler-rt/lib/msan/msan_interceptors.cc:990:3
    #4 0x7f570ff6bcb0 in killpg ??:?
    #5 0x7f570ff6bcb0 in ?? ??:0
    #6 0x7f57100336d3 in epoll_wait /build/eglibc-ripdx6/eglibc-2.19/misc/../sysdeps/unix/syscall-template.S:81:0
    #7 0x000000b92914 in __interceptor_epoll_wait /b/swarming/w/ir/kitchen-workdir/src/third_party/llvm/compiler-rt/lib/msan/msan_interceptors.cc:857:13
    #8 0x0000184d144b in epoll_dispatch ./../../base/third_party/libevent/epoll.c:198:8
    #9 0x0000184c484b in event_base_loop ./../../base/third_party/libevent/event.c:512:9
    #10 0x0000148a8ab8 in base::MessagePumpLibevent::Run(base::MessagePump::Delegate*) ./../../base/message_loop/message_pump_libevent.cc:247:9
    #11 0x0000146579c0 in base::RunLoop::Run() ./../../base/run_loop.cc:102:14
    #12 0x0000164f8b39 in content::DOMMessageQueue::WaitForMessage(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >*) ./../../content/public/test/browser_test_utils.cc:2118:14
    #13 0x0000164ecbd8 in content::(anonymous namespace)::ExecuteScriptHelper(content::RenderFrameHost*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, bool, int, std::__1::unique_ptr<base::Value, std::__1::default_delete<base::Value> >*) ./../../content/public/test/browser_test_utils.cc:193:26
    #14 0x0000164eb5bc in content::ExecuteScriptAndExtractString(content::ToRenderFrameHost const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >*) ./../../content/public/test/browser_test_utils.cc:1168:10
    #15 0x000000f20cd0 in RunScript ./../../chrome/browser/background_fetch/background_fetch_browsertest.cc:270:12
    #16 0x000000f20cd0 in BackgroundFetchBrowserTest::RunScriptAndCheckResultingMessage(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) ./../../chrome/browser/background_fetch/background_fetch_browsertest.cc:238:0
    #17 0x000000f231cf in BackgroundFetchBrowserTest_FetchFromServiceWorker_Test::RunTestOnMainThread() ./../../chrome/browser/background_fetch/background_fetch_browsertest.cc:563:3
    #18 0x0000164d833c in content::BrowserTestBase::ProxyRunTestOnMainThreadLoop() ./../../content/public/test/browser_test_base.cc:406:5
    #19 0x000014c6172f in Run ./../../base/callback.h:129:12
    #20 0x000014c6172f in ChromeBrowserMainParts::PreMainMessageLoopRunImpl() ./../../chrome/browser/chrome_browser_main.cc:2004:0
    #21 0x000014c5ce9c in ChromeBrowserMainParts::PreMainMessageLoopRun() ./../../chrome/browser/chrome_browser_main.cc:1388:18
    #22 0x000007db653e in chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun() ./../../chrome/browser/chromeos/chrome_browser_main_chromeos.cc:692:32
    #23 0x00000cc645fb in content::BrowserMainLoop::PreMainMessageLoopRun() ./../../content/browser/browser_main_loop.cc:1016:13
    #24 0x00000e376add in Run ./../../base/callback.h:129:12
    #25 0x00000e376add in content::StartupTaskRunner::RunAllTasksNow() ./../../content/browser/startup_task_runner.cc:43:0
    #26 0x00000cc5c03b in content::BrowserMainLoop::CreateStartupTasks() ./../../content/browser/browser_main_loop.cc:927:25
    #27 0x00000cc70b4a in content::BrowserMainRunnerImpl::Initialize(content::MainFunctionParams const&) ./../../content/browser/browser_main_runner_impl.cc:140:15
    #28 0x00000cc50a46 in content::BrowserMain(content::MainFunctionParams const&) ./../../content/browser/browser_main.cc:43:32
    #29 0x00001414e608 in RunBrowserProcessMain ./../../content/app/content_main_runner_impl.cc:536:10
    #30 0x00001414e608 in content::ContentMainRunnerImpl::Run(bool) ./../../content/app/content_main_runner_impl.cc:889:0
    #31 0x00001eae3e11 in service_manager::Main(service_manager::MainParams const&) ./../../services/service_manager/embedder/main.cc:472:29
    #32 0x000014145aa5 in content::ContentMain(content::ContentMainParams const&) ./../../content/app/content_main.cc:19:10
    #33 0x0000164d67ff in content::BrowserTestBase::SetUp() ./../../content/public/test/browser_test_base.cc:322:3
    #34 0x000014a92550 in InProcessBrowserTest::SetUp() ./../../chrome/test/base/in_process_browser_test.cc:251:20
    #35 0x000009531926 in testing::Test::Run() ./../../third_party/googletest/src/googletest/src/gtest.cc:0:0
    #36 0x0000095359cc in testing::TestInfo::Run() ./../../third_party/googletest/src/googletest/src/gtest.cc:2682:11
    #37 0x00000953748a in testing::TestCase::Run() ./../../third_party/googletest/src/googletest/src/gtest.cc:2800:28
    #38 0x00000956d9a5 in testing::internal::UnitTestImpl::RunAllTests() ./../../third_party/googletest/src/googletest/src/gtest.cc:5124:43
    #39 0x00000956c278 in testing::UnitTest::Run() ./../../third_party/googletest/src/googletest/src/gtest.cc:0:0
    #40 0x000014b080a1 in RUN_ALL_TESTS ./../../third_party/googletest/src/googletest/include/gtest/gtest.h:2331:46
    #41 0x000014b080a1 in base::TestSuite::Run() ./../../base/test/test_suite.cc:295:0
    #42 0x0000144df28d in ChromeTestSuiteRunner::RunTestSuite(int, char**) ./../../chrome/test/base/chrome_test_launcher.cc:68:21
    #43 0x000016589d2a in content::LaunchTests(content::TestLauncherDelegate*, unsigned long, int, char**) ./../../content/public/test/test_launcher.cc:645:31
    #44 0x0000144e09d9 in LaunchChromeTests(unsigned long, content::TestLauncherDelegate*, int, char**) ./../../chrome/test/base/chrome_test_launcher.cc:173:10
    #45 0x0000144df050 in main ./../../chrome/test/base/browser_tests_main_chromeos.cc:21:10
    #46 0x7f570ff56f45 in __libc_start_main /build/eglibc-ripdx6/eglibc-2.19/csu/libc-start.c:287:0
    #47 0x000000b6be0a in _start ??:0:0
[ RUN      ] BackgroundFetchBrowserTest.FetchFromServiceWorker

In https://cs.chromium.org/chromium/src/chrome/test/data/background_fetch/background_fetch.js?sq=package:chromium&dr&g=0&l=139 (the only call to postMessage)

This makes me thing installation of service worker didn't work properly in sw.js:
https://cs.chromium.org/chromium/src/chrome/test/data/background_fetch/sw.js

I wonder if that could be due to scoping when we skip waiting:
// Service Worker initialization listeners.
self.addEventListener('install', e => e.waitUntil(skipWaiting()));
self.addEventListener('activate', e => e.waitUntil(clients.claim()));

I'd try to pre-pend "self." to both calls, but I am not an expert. Either way this test fails a lot.