WorkerThread::m_workerGlobalScope can be accessed both main/worker threads, but it's nullified without a lock on the worker thread. This causes TSan failures as follow:

(This was originally reported at https://bugs.chromium.org/p/chromium/issues/detail?id=345240#c26)

[ RUN      ] ServiceWorkerBlackBoxBrowserTest.Registration
[7664:7664:0818/021615:37476997980:WARNING:audio_manager.cc(317)] Multiple instances of AudioManager detected
[7664:7664:0818/021615:37476998140:WARNING:audio_manager.cc(278)] Multiple instances of AudioManager detected
Xlib:  extension "RANDR" missing on display ":9".
[7664:7755:0818/021616:37478128060:WARNING:embedded_test_server.cc(202)] Request not handled. Returning 404: /does/not/exist
==================
WARNING: ThreadSanitizer: data race (pid=7798)
  Write of size 8 at 0x7d340000b5a8 by thread T9:
    #0 blink::PersistentBase<blink::WorkerOrWorkletGlobalScope, (blink::WeaknessPersistentConfiguration)0, (blink::CrossThreadnessPersistentConfiguration)0>::assign(blink::WorkerOrWorkletGlobalScope*) third_party/WebKit/Source/platform/heap/Persistent.h:172:19 (content_browsertests+0x000004dd8bf4)
    #1 operator= third_party/WebKit/Source/platform/heap/Persistent.h:119:9 (content_browsertests+0x000004dd6650)
    #2 operator= third_party/WebKit/Source/platform/heap/Persistent.h:300 (content_browsertests+0x000004dd6650)
    #3 blink::WorkerThread::performShutdownOnWorkerThread() third_party/WebKit/Source/core/workers/WorkerThread.cpp:596 (content_browsertests+0x000004dd6650)
    #4 Invoke<blink::WorkerThread *> base/bind_internal.h:214:12 (content_browsertests+0x000004dd8015)
    #5 MakeItSo<void (blink::WorkerThread::*const &)(), blink::WorkerThread *> base/bind_internal.h:283 (content_browsertests+0x000004dd8015)
    #6 RunImpl<void (blink::WorkerThread::*const &)(), const std::__1::tuple<WTF::UnretainedWrapper<blink::WorkerThread, WTF::FunctionThreadAffinity::CrossThreadAffinity> > &, 0> base/bind_internal.h:346 (content_browsertests+0x000004dd8015)
    #7 base::internal::Invoker<base::internal::BindState<void (blink::WorkerThread::*)(), WTF::UnretainedWrapper<blink::WorkerThread, (WTF::FunctionThreadAffinity)0> >, void ()>::Run(base::internal::BindStateBase*) base/bind_internal.h:324 (content_browsertests+0x000004dd8015)
    #8 Run base/callback.h:388:12 (content_browsertests+0x000003e9c752)
    #9 operator() third_party/WebKit/Source/wtf/Functional.h:229 (content_browsertests+0x000003e9c752)
    #10 blink::CrossThreadTask::run() third_party/WebKit/Source/platform/WebTaskRunner.cpp:38 (content_browsertests+0x000003e9c752)
    #11 blink::scheduler::WebTaskRunnerImpl::runTask(std::__1::unique_ptr<blink::WebTaskRunner::Task, std::__1::default_delete<blink::WebTaskRunner::Task> >) third_party/WebKit/Source/platform/scheduler/child/web_task_runner_impl.cc:71:9 (content_browsertests+0x000003eded4d)
    #12 Invoke<std::__1::unique_ptr<blink::WebTaskRunner::Task, std::__1::default_delete<blink::WebTaskRunner::Task> > > base/bind_internal.h:164:12 (content_browsertests+0x000003edf1b1)
    #13 MakeItSo<void (*const &)(std::__1::unique_ptr<blink::WebTaskRunner::Task, std::__1::default_delete<blink::WebTaskRunner::Task> >), std::__1::unique_ptr<blink::WebTaskRunner::Task, std::__1::default_delete<blink::WebTaskRunner::Task> > > base/bind_internal.h:283 (content_browsertests+0x000003edf1b1)
    #14 RunImpl<void (*const &)(std::__1::unique_ptr<blink::WebTaskRunner::Task, std::__1::default_delete<blink::WebTaskRunner::Task> >), const std::__1::tuple<base::internal::PassedWrapper<std::__1::unique_ptr<blink::WebTaskRunner::Task, std::__1::default_delete<blink::WebTaskRunner::Task> > > > &, 0> base/bind_internal.h:346 (content_browsertests+0x000003edf1b1)
    #15 base::internal::Invoker<base::internal::BindState<void (*)(std::__1::unique_ptr<blink::WebTaskRunner::Task, std::__1::default_delete<blink::WebTaskRunner::Task> >), base::internal::PassedWrapper<std::__1::unique_ptr<blink::WebTaskRunner::Task, std::__1::default_delete<blink::WebTaskRunner::Task> > > >, void ()>::Run(base::internal::BindStateBase*) base/bind_internal.h:324 (content_browsertests+0x000003edf1b1)
    #16 Run base/callback.h:388:12 (content_browsertests+0x00000287ac49)
    #17 base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask const&) base/debug/task_annotator.cc:54 (content_browsertests+0x00000287ac49)
    #18 blink::scheduler::TaskQueueManager::ProcessTaskFromWorkQueue(blink::scheduler::internal::WorkQueue*, blink::scheduler::internal::TaskQueueImpl::Task*) third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc:315:19 (content_browsertests+0x000003ef8ec5)
    #19 blink::scheduler::TaskQueueManager::DoWork(base::TimeTicks, bool) third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc:218:13 (content_browsertests+0x000003ef7356)
    #20 Invoke<const base::WeakPtr<blink::scheduler::TaskQueueManager> &, const base::TimeTicks &, const bool &> base/bind_internal.h:214:12 (content_browsertests+0x000003efa03f)
...

  Previous read of size 8 at 0x7d340000b5a8 by main thread (mutexes: write M2799):
    #0 operator blink::WorkerOrWorkletGlobalScope * third_party/WebKit/Source/platform/heap/Persistent.h:106:34 (content_browsertests+0x000004dd426a)
    #1 blink::WorkerThread::terminateInternal(blink::WorkerThread::TerminationMode) third_party/WebKit/Source/core/workers/WorkerThread.cpp:368 (content_browsertests+0x000004dd426a)
    #2 blink::WorkerThread::terminateAndWait() third_party/WebKit/Source/core/workers/WorkerThread.cpp:216:5 (content_browsertests+0x000004dd4b1b)
    #3 blink::WebEmbeddedWorkerImpl::~WebEmbeddedWorkerImpl() third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.cpp:111:25 (content_browsertests+0x000003f9fdba)
    #4 blink::WebEmbeddedWorkerImpl::~WebEmbeddedWorkerImpl() third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.cpp:106:1 (content_browsertests+0x000003fa05d9)
    #5 operator() buildtools/third_party/libc++/trunk/include/memory:2529:13 (content_browsertests+0x000003ba6789)
    #6 reset buildtools/third_party/libc++/trunk/include/memory:2735 (content_browsertests+0x000003ba6789)
    #7 ~unique_ptr buildtools/third_party/libc++/trunk/include/memory:2703 (content_browsertests+0x000003ba6789)
    #8 ~WorkerWrapper content/renderer/service_worker/embedded_worker_dispatcher.cc:38 (content_browsertests+0x000003ba6789)
    #9 IDMap<content::EmbeddedWorkerDispatcher::WorkerWrapper, (IDMapOwnershipSemantics)1, int>::Releaser<(IDMapOwnershipSemantics)1, 0>::release_all(std::__1::unordered_map<int, content::EmbeddedWorkerDispatcher::WorkerWrapper*, base_hash::hash<int>, std::__1::equal_to<int>, std::__1::allocator<std::__1::pair<int const, content::EmbeddedWorkerDispatcher::WorkerWrapper*> > >*) base/id_map.h:250 (content_browsertests+0x000003ba6789)
    #10 ~IDMap base/id_map.h:59:5 (content_browsertests+0x000003ba4ae3)
...

  Location is heap block of size 200 at 0x7d340000b530 allocated by main thread:
    #0 operator new(unsigned long) <null> (content_browsertests+0x00000050f2c2)
    #1 blink::ServiceWorkerThread::create(WTF::PassRefPtr<blink::WorkerLoaderProxy>, blink::WorkerReportingProxy&) third_party/WebKit/Source/modules/serviceworkers/ServiceWorkerThread.cpp:43:23 (content_browsertests+0x000005330339)
    #2 blink::WebEmbeddedWorkerImpl::startWorkerThread() third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.cpp:451:22 (content_browsertests+0x000003fa15ef)
    #3 blink::WebEmbeddedWorkerImpl::onScriptLoaderFinished() third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.cpp:398:5 (content_browsertests+0x000003fa2b9a)
    #4 Invoke<blink::WebEmbeddedWorkerImpl *> base/bind_internal.h:214:12 (content_browsertests+0x000003fa5ed5)
    #5 MakeItSo<void (blink::WebEmbeddedWorkerImpl::*const &)(), blink::WebEmbeddedWorkerImpl *> base/bind_internal.h:283 (content_browsertests+0x000003fa5ed5)
    #6 RunImpl<void (blink::WebEmbeddedWorkerImpl::*const &)(), const std::__1::tuple<WTF::UnretainedWrapper<blink::WebEmbeddedWorkerImpl, WTF::FunctionThreadAffinity::SameThreadAffinity> > &, 0> base/bind_internal.h:346 (content_browsertests+0x000003fa5ed5)
    #7 base::internal::Invoker<base::internal::BindState<void (blink::WebEmbeddedWorkerImpl::*)(), WTF::UnretainedWrapper<blink::WebEmbeddedWorkerImpl, (WTF::FunctionThreadAffinity)1> >, void ()>::Run(base::internal::BindStateBase*) base/bind_internal.h:324 (content_browsertests+0x000003fa5ed5)
    #8 Run base/callback.h:388:12 (content_browsertests+0x000004dd2841)