The test seems quite flaky on mac_chromium_rel_ng. Failure looks similar to issue 889304.

Flakiness dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=HasBeforeUnloadHandlerTest.EmptyBeforeUnloadDetected

Recent failure example:
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/mac_chromium_rel_ng/178853

Stack:
====
[ RUN      ] HasBeforeUnloadHandlerTest.EmptyBeforeUnloadDetected
[23653:6403:1106/063405.099133:WARNING:notification_platform_bridge_mac.mm(512)] AlertNotificationService: XPC connection invalidated.
[23653:775:1106/063405.314742:ERROR:delegated_frame_host.cc(159)] Not implemented reached in void content::DelegatedFrameHost::SetNeedsBeginFrames(bool)
[23661:775:1106/063405.351693:WARNING:vt_video_decode_accelerator_mac.cc(193)] Failed to create VTDecompressionSession: Error Domain=NSOSStatusErrorDomain Code=-8973 "codecOpenErr" (-8973)
[23661:775:1106/063405.352260:WARNING:vt_video_decode_accelerator_mac.cc(215)] Hardware decoding with VideoToolbox is not supported
[23661:775:1106/063405.539249:ERROR:vt_video_encode_accelerator_mac.cc(513)]  VTCompressionSessionCreate failed: -12908
[23653:775:1106/063405.703568:WARNING:gaia_auth_fetcher.cc(861)] Could not reach Google Accounts servers: errno -11
[23661:775:1106/063406.264787:WARNING:ca_layer_tree_coordinator.mm(55)] Blank frame: No overlays or CALayers
[23653:129575:1106/063406.512279:WARNING:embedded_test_server.cc(239)] Request not handled. Returning 404: /favicon.ico
[23653:775:1106/063406.539016:FATAL:browser_tabstrip.cc(76)] Check failed: false. CloseWebContents called for tab not in our strip
0   browser_tests                       0x000000011232a80f base::debug::StackTrace::StackTrace(unsigned long) + 31
1   browser_tests                       0x00000001122270df logging::LogMessage::~LogMessage() + 223
2   browser_tests                       0x00000001162a6ecf chrome::CloseWebContents(Browser*, content::WebContents*, bool) + 143
3   browser_tests                       0x000000010fca6f01 bool IPC::MessageT<ViewHostMsg_ClosePage_ACK_Meta, std::__1::tuple<>, void>::Dispatch<content::RenderViewHostImpl, content::RenderViewHostImpl, void, void (content::RenderViewHostImpl::*)()>(IPC::Message const*, content::RenderViewHostImpl*, content::RenderViewHostImpl*, void*, void (content::RenderViewHostImpl::*)()) + 97
4   browser_tests                       0x000000010fca6354 content::RenderViewHostImpl::OnMessageReceived(IPC::Message const&) + 548
5   browser_tests                       0x000000010fc876cb content::RenderProcessHostImpl::OnMessageReceived(IPC::Message const&) + 491
6   browser_tests                       0x0000000113b39cf1 IPC::ChannelProxy::Context::OnDispatchMessage(IPC::Message const&) + 129
7   browser_tests                       0x0000000113b3c55b base::internal::Invoker<base::internal::BindState<void (IPC::ChannelProxy::Context::*)(IPC::Message const&), scoped_refptr<IPC::ChannelProxy::Context>, IPC::Message>, void ()>::Run(base::internal::BindStateBase*) + 139
8   browser_tests                       0x0000000112210e86 base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask*) + 310
9   browser_tests                       0x000000011224267f base::MessageLoopImpl::RunTask(base::PendingTask*) + 271
10  browser_tests                       0x0000000112242c23 base::MessageLoopImpl::DoWork() + 387
11  browser_tests                       0x0000000112244e2a base::MessagePumpCFRunLoopBase::RunWork() + 42
12  browser_tests                       0x000000011222a07a base::mac::CallWithEHFrame(void () block_pointer) + 10
13  browser_tests                       0x000000011224474f base::MessagePumpCFRunLoopBase::RunWorkSource(void*) + 63
14  CoreFoundation                      0x00007fff34b9bd81 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
15  CoreFoundation                      0x00007fff34c5365c __CFRunLoopDoSource0 + 108
16  CoreFoundation                      0x00007fff34b7ed30 __CFRunLoopDoSources0 + 208
17  CoreFoundation                      0x00007fff34b7e1ad __CFRunLoopRun + 1293
18  CoreFoundation                      0x00007fff34b7da07 CFRunLoopRunSpecific + 487
19  HIToolbox                           0x00007fff33e59d96 RunCurrentEventLoopInMode + 286
20  HIToolbox                           0x00007fff33e59b06 ReceiveNextEventCommon + 613
21  HIToolbox                           0x00007fff33e59884 _BlockUntilNextEventMatchingListInModeWithFilter + 64
22  AppKit                              0x00007fff3210ba73 _DPSNextEvent + 2085
23  AppKit                              0x00007fff328a1e34 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044
24  browser_tests                       0x000000011242eeb0 __71-[BrowserCrApplication nextEventMatchingMask:untilDate:inMode:dequeue:]_block_invoke + 64
25  browser_tests                       0x000000011222a07a base::mac::CallWithEHFrame(void () block_pointer) + 10
26  browser_tests                       0x000000011242ede4 -[BrowserCrApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 164
27  AppKit                              0x00007fff32100885 -[NSApplication run] + 764
28  browser_tests                       0x0000000112245edc base::MessagePumpNSApplication::DoRun(base::MessagePump::Delegate*) + 364
29  browser_tests                       0x000000011224415e base::MessagePumpCFRunLoopBase::Run(base::MessagePump::Delegate*) + 110
30  browser_tests                       0x0000000112242224 base::MessageLoopImpl::Run(bool) + 132
31  browser_tests                       0x000000011227dda9 base::RunLoop::Run() + 249
32  browser_tests                       0x0000000112385cd5 InProcessBrowserTest::QuitBrowsers() + 309
33  browser_tests                       0x0000000112385b58 InProcessBrowserTest::PostRunTestOnMainThread() + 40
34  browser_tests                       0x0000000112bb28f0 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop() + 1056
35  browser_tests                       0x0000000112434609 ChromeBrowserMainParts::PreMainMessageLoopRunImpl() + 4921
36  browser_tests                       0x00000001124331d7 ChromeBrowserMainParts::PreMainMessageLoopRun() + 55
37  browser_tests                       0x000000010f803e07 content::BrowserMainLoop::PreMainMessageLoopRun() + 55
38  browser_tests                       0x000000010fe0f0f5 content::StartupTaskRunner::RunAllTasksNow() + 117
39  browser_tests                       0x000000010f8026fa content::BrowserMainLoop::CreateStartupTasks() + 890
40  browser_tests                       0x000000010f806a40 content::BrowserMainRunnerImpl::Initialize(content::MainFunctionParams const&) + 96
41  browser_tests                       0x000000010f800422 content::BrowserMain(content::MainFunctionParams const&) + 210
42  browser_tests                       0x0000000111e167cb content::ContentMainRunnerImpl::RunServiceManager(content::MainFunctionParams&, bool) + 267
43  browser_tests                       0x0000000111e1668e content::ContentMainRunnerImpl::Run(bool) + 446
44  browser_tests                       0x00000001159e4bae service_manager::Main(service_manager::MainParams const&) + 3166
45  browser_tests                       0x0000000111e15784 content::ContentMain(content::ContentMainParams const&) + 68
46  browser_tests                       0x0000000112bb22a3 content::BrowserTestBase::SetUp() + 3603
47  browser_tests                       0x0000000112384cee InProcessBrowserTest::SetUp() + 606
48  browser_tests                       0x000000010e33f09d testing::Test::Run() + 109
49  browser_tests                       0x000000010e33fd90 testing::TestInfo::Run() + 288
50  browser_tests                       0x000000010e3403b7 testing::TestCase::Run() + 247
51  browser_tests                       0x000000010e34d5f7 testing::internal::UnitTestImpl::RunAllTests() + 855
52  browser_tests                       0x000000010e34d27d testing::UnitTest::Run() + 109
53  browser_tests                       0x00000001123aaca6 base::TestSuite::Run() + 166
54  browser_tests                       0x00000001121f2d7d ChromeTestSuiteRunner::RunTestSuite(int, char**) + 45
55  browser_tests                       0x0000000112bee68e content::LaunchTests(content::TestLauncherDelegate*, unsigned long, int, char**) + 622
56  browser_tests                       0x00000001121f332e LaunchChromeTests(unsigned long, content::TestLauncherDelegate*, int, char**) + 494
57  browser_tests                       0x00000001121f2afe main + 94
58  libdyld.dylib                       0x00007fff5ca3a015 start + 1
59  ???                                 0x000000000000000b 0x0 + 11