Disallow cross-thread Persistent<> read access.

third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.h

Index: third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.h
diff --git a/third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.h b/third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.h
index 71295710b7d0f3a234fa9feeabc0dba07f65975c..e407dd80db180cae4d7a44c9238b650002ec4a22 100644
--- a/third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.h
+++ b/third_party/WebKit/Source/web/WebEmbeddedWorkerImpl.h
@@ -122,7 +122,9 @@ class WebEmbeddedWorkerImpl final : public WebEmbeddedWorker,
   // 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;
@@ -134,7 +136,11 @@ class WebEmbeddedWorkerImpl final : public WebEmbeddedWorker,
   // 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;