Disallow cross-thread Persistent<> read access.

third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.h

 /*
  * Copyright (C) 2013 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 104 matching lines @@
   // to WorkerContext.
   std::unique_ptr<WebWorkerContentSettingsClientProxy> m_contentSettingsClient;
 
   // We retain ownership of this one which is for use on the
   // main thread only.
   std::unique_ptr<WebServiceWorkerNetworkProvider> m_networkProvider;
 
   // Kept around only while main script loading is ongoing.
   RefPtr<WorkerScriptLoader> m_mainScriptLoader;
 
-  Persistent<ParentFrameTaskRunners> m_mainThreadTaskRunners;
+  // Owned by the main thread, but will be accessed by the worker when
+  // posting tasks.
+  CrossThreadPersistent<ParentFrameTaskRunners> m_mainThreadTaskRunners;
 
   std::unique_ptr<WorkerThread> m_workerThread;
   RefPtr<WorkerLoaderProxy> m_loaderProxy;
   Persistent<ServiceWorkerGlobalScopeProxy> m_workerGlobalScopeProxy;
   Persistent<WorkerInspectorProxy> m_workerInspectorProxy;
 
   // 'shadow page' - created to proxy loading requests from the worker.
   // Both WebView and WebFrame objects are close()'ed (where they're
   // deref'ed) when this EmbeddedWorkerImpl is destructed, therefore they
   // are guaranteed to exist while this object is around.
   WebView* m_webView;
-  Persistent<WebLocalFrameImpl> m_mainFrame;
+
+  // Accessed cross-thread when worker thread posts tasks to the parent.
+  //
+  // TODO: avoid reaching into the local frame object when posting.
+  CrossThreadPersistent<WebLocalFrameImpl> m_mainFrame;
 
   bool m_loadingShadowPage;
   bool m_askedToTerminate;
 
   enum WaitingForDebuggerState { WaitingForDebugger, NotWaitingForDebugger };
 
   enum {
     DontPauseAfterDownload,
     DoPauseAfterDownload,
     IsPausedAfterDownload
   } m_pauseAfterDownloadState;
 
   WaitingForDebuggerState m_waitingForDebuggerState;
 };
 
 }  // namespace blink
 
 #endif  // WebEmbeddedWorkerImpl_h