Disallow cross-thread Persistent<> read access.

third_party/WebKit/Source/core/workers/WorkerThread.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 291 matching lines @@
   CrossThreadPersistent<WorkerOrWorkletGlobalScope> m_globalScope;
   CrossThreadPersistent<WorkerInspectorController> m_workerInspectorController;
 
   // Signaled when the thread completes termination on the worker thread.
   std::unique_ptr<WaitableEvent> m_shutdownEvent;
 
   // Used to cancel a scheduled forcible termination task. See
   // mayForciblyTerminateExecution() for details.
   TaskHandle m_forcibleTerminationTaskHandle;
 
-  Persistent<WorkerThreadLifecycleContext> m_workerThreadLifecycleContext;
+  // Created on the main thread heap, but will be accessed cross-thread
+  // when worker thread posts tasks.
+  CrossThreadPersistent<WorkerThreadLifecycleContext>
+      m_workerThreadLifecycleContext;
 };
 
 }  // namespace blink
 
 #endif  // WorkerThread_h