Disallowed PassRefPtr copies across threads.

third_party/WebKit/Source/platform/CrossThreadCopier.h

 /*
  * Copyright (C) 2009, 2010 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 73 matching lines @@
     STATIC_ONLY(CrossThreadCopierBase);
 };
 
 template <typename T>
 struct CrossThreadCopier : public CrossThreadCopierBase<T, std::is_arithmetic<T>::value || std::is_enum<T>::value> {
     STATIC_ONLY(CrossThreadCopier);
 };
 
 // CrossThreadCopier specializations follow.
 template <typename T>
 struct CrossThreadCopier<PassRefPtr<T>> : public CrossThreadCopierPassThrough<PassRefPtr<T>> {

[Yuta Kitamura, 2016/09/23 07:33:35]

You probably should remove this inheritance and define Type yourself,
since this isn't really the usual "pass through" copy.

     STATIC_ONLY(CrossThreadCopier);
     static_assert(WTF::IsSubclassOfTemplate<T, ThreadSafeRefCounted>::value, "PassRefPtr<T> can be passed across threads only if T is ThreadSafeRefCounted.");
+    static PassRefPtr<T> copy(PassRefPtr<T>&& pointer)
+    {
+        return std::move(pointer);
+    }
 };
 template <typename T>
 struct CrossThreadCopier<RefPtr<T>> : public CrossThreadCopierPassThrough<RefPtr<T>> {
     STATIC_ONLY(CrossThreadCopier);
     static_assert(WTF::IsSubclassOfTemplate<T, ThreadSafeRefCounted>::value, "RefPtr<T> can be passed across threads only if T is ThreadSafeRefCounted.");
 };
 template <typename T>
 struct CrossThreadCopier<sk_sp<T>> : public CrossThreadCopierPassThrough<sk_sp<T>> {
     STATIC_ONLY(CrossThreadCopier);
     static_assert(std::is_base_of<SkRefCnt, T>::value, "sk_sp<T> can be passed across threads only if T is SkRefCnt.");
@@ skipping 85 matching lines @@
 template <>
 struct CrossThreadCopier<ResourceResponse> {
     STATIC_ONLY(CrossThreadCopier);
     typedef WTF::PassedWrapper<std::unique_ptr<CrossThreadResourceResponseData>> Type;
     PLATFORM_EXPORT static Type copy(const ResourceResponse&);
 };
 
 } // namespace blink
 
 #endif // CrossThreadCopier_h