Disallow cross-thread Persistent<> read access.

third_party/WebKit/Source/platform/graphics/CompositorMutatorClient.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CompositorMutatorClient_h
 #define CompositorMutatorClient_h
 
 #include "platform/PlatformExport.h"
 #include "platform/heap/Handle.h"
 #include "public/platform/WebCompositorMutatorClient.h"
@@ skipping 18 matching lines @@
   void SetClient(cc::LayerTreeMutatorClient*) override;
   base::Closure TakeMutations() override;
 
   CompositorMutator* mutator() { return m_mutator.get(); }
 
   void setMutationsForTesting(std::unique_ptr<CompositorMutations>);
 
  private:
   cc::LayerTreeMutatorClient* m_client;
   CompositorMutationsTarget* m_mutationsTarget;
-  Persistent<CompositorMutator> m_mutator;
+  // Accessed by main and compositor threads.
+  CrossThreadPersistent<CompositorMutator> m_mutator;
   std::unique_ptr<CompositorMutations> m_mutations;
 };
 
 }  // namespace blink
 
 #endif  // CompositorMutatorClient_h