Disallow cross-thread Persistent<> read access.

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

Index: third_party/WebKit/Source/core/workers/WorkerThread.h
diff --git a/third_party/WebKit/Source/core/workers/WorkerThread.h b/third_party/WebKit/Source/core/workers/WorkerThread.h
index e7a8629e66aee570fea13ab1012268faffeafb6f..cb6ece4c641d417f27ff2da8c01c2e4fbcfd04da 100644
--- a/third_party/WebKit/Source/core/workers/WorkerThread.h
+++ b/third_party/WebKit/Source/core/workers/WorkerThread.h
@@ -309,7 +309,10 @@ class CORE_EXPORT WorkerThread : public WebThread::TaskObserver {
   // 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