https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=content_browsertests&tests=DownloadContentTest.ForceDownloadMessageRfc822Page shows a couple of failures for this test on the TSAN bot:

https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20TSan%20Tests/20712
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20TSan%20Tests/20678 (succeeded on a retry)

[ RUN      ] DownloadContentTest.ForceDownloadMessageRfc822Page
Xlib:  extension "RANDR" missing on display ":99".
DevTools listening on ws://127.0.0.1:39325/devtools/browser/5f0cc992-cbea-490e-90af-498e1cecd041
==================
WARNING: ThreadSanitizer: data race (pid=11191)
  Write of size 8 at 0x7b100001c8c8 by main thread:
    #0 SetBrowser content/public/common/content_client.cc:22:24 (content_browsertests+0x2949505)
    #1 content::SetBrowserClientForTesting(content::ContentBrowserClient*) content/public/common/content_client.cc:48 (content_browsertests+0x2949505)
    #2 content::DownloadContentTest_ForceDownloadMessageRfc822Page_Test::RunTestOnMainThread() content/browser/download/download_browsertest.cc:3403:3 (content_browsertests+0x184a18b)
...
  Previous read of size 8 at 0x7b100001c8c8 by thread T15:
    #0 browser content/public/common/content_client.h:78:44 (content_browsertests+0x4343e4e)
    #1 content::internal::CreateDefaultPosixFilesToMap(int, mojo::edk::PlatformHandle const&, bool, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, base::CommandLine*) content/browser/child_process_launcher_helper_posix.cc:96 (content_browsertests+0x4343e4e)
    #2 content::internal::ChildProcessLauncherHelper::GetFilesToMap() content/browser/child_process_launcher_helper_linux.cc:41:10 (content_browsertests+0x3d84a9d)
    #3 content::internal::ChildProcessLauncherHelper::LaunchOnLauncherThread() content/browser/child_process_launcher_helper.cc:110:60 (content_browsertests+0x3d83e7a)
...
  Location is heap block of size 64 at 0x7b100001c8c0 allocated by main thread:
    #0 operator new(unsigned long) /b/build/slave/linux_upload_clang/build/src/third_party/llvm/compiler-rt/lib/tsan/rtl/tsan_new_delete.cc:57:3 (content_browsertests+0x1711349)
    #1 content::ShellMainDelegate::BasicStartupComplete(int*) content/shell/app/shell_main_delegate.cc (content_browsertests+0x4737d4a)
    #2 content::ContentMainRunnerImpl::Initialize(content::ContentMainParams const&) content/app/content_main_runner.cc:729:33 (content_browsertests+0x3bdf51e)
    #3 content::ContentServiceManagerMainDelegate::Initialize(service_manager::MainDelegate::InitializeParams const&) content/app/content_service_manager_main_delegate.cc:37:32 (content_browsertests+0x2d23207)
    #4 service_manager::Main(service_manager::MainParams const&) services/service_manager/embedder/main.cc:409:29 (content_browsertests+0x67c8d25)
    #5 content::ContentMain(content::ContentMainParams const&) content/app/content_main.cc:19:10 (content_browsertests+0x2d23c0e)
...
  Thread T15 'TaskSchedulerSi' (tid=11259, running) created by thread T2 at:
    #0 pthread_create /b/build/slave/linux_upload_clang/build/src/third_party/llvm/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:965:3 (content_browsertests+0x16a7785)
    #1 base::(anonymous namespace)::CreateThread(unsigned long, bool, base::PlatformThread::Delegate*, base::PlatformThreadHandle*, base::ThreadPriority) base/threading/platform_thread_posix.cc:115:13 (content_browsertests+0x4c472c7)
    #2 base::PlatformThread::CreateWithPriority(unsigned long, base::PlatformThread::Delegate*, base::PlatformThreadHandle*, base::ThreadPriority) base/threading/platform_thread_posix.cc:200:10 (content_browsertests+0x4c471c5)
    #3 base::internal::SchedulerWorker::Start() base/task_scheduler/scheduler_worker.cc:67:3 (content_browsertests+0x4bd307f)
    #4 CreateTaskRunnerWithTraitsImpl<base::internal::(anonymous namespace)::SchedulerWorkerDelegate> base/task_scheduler/scheduler_single_thread_task_runner_manager.cc:490:13 (content_browsertests+0x4bd4867)
    #5 base::internal::SchedulerSingleThreadTaskRunnerManager::CreateSingleThreadTaskRunnerWithTraits(base::TaskTraits const&, base::SingleThreadTaskRunnerThreadMode) base/task_scheduler/scheduler_single_thread_task_runner_manager.cc:426 (content_browsertests+0x4bd4867)
    #6 base::internal::TaskSchedulerImpl::CreateSingleThreadTaskRunnerWithTraits(base::TaskTraits const&, base::SingleThreadTaskRunnerThreadMode) base/task_scheduler/task_scheduler_impl.cc:155:8 (content_browsertests+0x4bcb27c)
    #7 base::CreateSingleThreadTaskRunnerWithTraits(base::TaskTraits const&, base::SingleThreadTaskRunnerThreadMode) base/task_scheduler/post_task.cc:115:40 (content_browsertests+0x4bc979c)
    #8 Create base/task_scheduler/lazy_task_runner.cc:54:10 (content_browsertests+0x4bc8e41)
    #9 base::internal::LazyTaskRunner<base::SingleThreadTaskRunner, false>::CreateRaw(void*) base/task_scheduler/lazy_task_runner.cc:72 (content_browsertests+0x4bc8e41)
    #10 GetOrCreateLazyPointer<base::SingleThreadTaskRunner> base/lazy_instance_helpers.h:83:48 (content_browsertests+0x4bc8daa)
    #11 base::internal::LazyTaskRunner<base::SingleThreadTaskRunner, false>::Get() base/task_scheduler/lazy_task_runner.cc:92 (content_browsertests+0x4bc8daa)
    #12 GetProcessLauncherTaskRunner content/browser/child_process_launcher_helper.cc:219:31 (content_browsertests+0x3d83cdc)
    #13 content::internal::ChildProcessLauncherHelper::StartLaunchOnClientThread() content/browser/child_process_launcher_helper.cc:99 (content_browsertests+0x3d83cdc)
    #14 content::ChildProcessLauncher::ChildProcessLauncher(std::__1::unique_ptr<content::SandboxedProcessLauncherDelegate, std::__1::default_delete<content::SandboxedProcessLauncherDelegate> >, std::__1::unique_ptr<base::CommandLine, std::__1::default_delete<base::CommandLine> >, int, content::ChildProcessLauncher::Client*, std::__1::unique_ptr<mojo::edk::OutgoingBrokerClientInvitation, std::__1::default_delete<mojo::edk::OutgoingBrokerClientInvitation> >, base::RepeatingCallback<void (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)> const&, bool) content/browser/child_process_launcher.cc:48:12 (content_browsertests+0x3d8315e)
    #15 content::BrowserChildProcessHostImpl::Launch(std::__1::unique_ptr<content::SandboxedProcessLauncherDelegate, std::__1::default_delete<content::SandboxedProcessLauncherDelegate> >, std::__1::unique_ptr<base::CommandLine, std::__1::default_delete<base::CommandLine> >, bool) content/browser/browser_child_process_host_impl.cc:268:28 (content_browsertests+0x3d222dc)
    #16 content::GpuProcessHost::LaunchGpuProcess() content/browser/gpu/gpu_process_host.cc:1327:13 (content_browsertests+0x3f54ced)
...