Disallow cross-thread Persistent<> read access.

third_party/WebKit/Source/web/WebSharedWorkerImpl.h

 /*
  * Copyright (C) 2009 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 139 matching lines @@
   void postMessageToPageInspectorOnMainThread(const String& message);
 
   // WorkerLoaderProxyProvider
   void postTaskToLoader(const WebTraceLocation&,
                         std::unique_ptr<ExecutionContextTask>) override;
   void postTaskToWorkerGlobalScope(
       const WebTraceLocation&,
       std::unique_ptr<WTF::CrossThreadClosure>) override;
 
   // 'shadow page' - created to proxy loading requests from the worker.
-  Persistent<ExecutionContext> m_loadingDocument;
+  // Will be accessed by worker thread when posting tasks.
+  //
+  // TODO: it is undesirable to keep a cross-thread reference to this
+  // document; avoid reaching into the document when posting.
+  CrossThreadPersistent<ExecutionContext> m_loadingDocument;
   WebView* m_webView;
   Persistent<WebLocalFrameImpl> m_mainFrame;
   bool m_askedToTerminate;
 
   // This one is bound to and used only on the main thread.
   std::unique_ptr<WebServiceWorkerNetworkProvider> m_networkProvider;
 
   Persistent<WorkerInspectorProxy> m_workerInspectorProxy;
 
-  Persistent<ParentFrameTaskRunners> m_parentFrameTaskRunners;
+  // Owned by the main thread, but will be accessed by the worker.
+  CrossThreadPersistent<ParentFrameTaskRunners> m_parentFrameTaskRunners;
 
   std::unique_ptr<WorkerThread> m_workerThread;
 
   WebSharedWorkerClient* m_client;
 
   bool m_pauseWorkerContextOnStart;
   bool m_isPausedOnStart;
 
   // Kept around only while main script loading is ongoing.
   RefPtr<WorkerScriptLoader> m_mainScriptLoader;
 
   RefPtr<WorkerLoaderProxy> m_loaderProxy;
 
   WebURL m_url;
   WebString m_name;
   WebAddressSpace m_creationAddressSpace;
 };
 
 }  // namespace blink
 
 #endif  // WebSharedWorkerImpl_h