Disallow cross-thread Persistent<> read access.

third_party/WebKit/Source/core/testing/DummyPageHolder.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 70 matching lines @@
   Document& document() const;
 
  private:
   DummyPageHolder(const IntSize& initialViewSize,
                   Page::PageClients*,
                   FrameLoaderClient*,
                   FrameSettingOverrideFunction settingOverrider,
                   InterfaceProvider* = nullptr);
 
   Persistent<Page> m_page;
-  Persistent<LocalFrame> m_frame;
+
+  // The LocalFrame is accessed from worker threads by unit tests
+  // (WorkerThreadableLoaderTest), hence we need to allow cross-thread
+  // usage of |m_frame|.
+  //
+  // TODO: rework the tests to not require cross-thread access.
+  CrossThreadPersistent<LocalFrame> m_frame;
 
   Persistent<FrameLoaderClient> m_frameLoaderClient;
 };
 
 }  // namespace blink
 
 #endif  // DummyPageHolder_h