Disallowed PassRefPtr copies across threads.

third_party/WebKit/Source/platform/CrossThreadCopier.h

Index: third_party/WebKit/Source/platform/CrossThreadCopier.h
diff --git a/third_party/WebKit/Source/platform/CrossThreadCopier.h b/third_party/WebKit/Source/platform/CrossThreadCopier.h
index 8d49eef467e90d7d648c6e21c75b6adb578ba960..f346ced979da1c7b879fd826abf4586b05f13030 100644
--- a/third_party/WebKit/Source/platform/CrossThreadCopier.h
+++ b/third_party/WebKit/Source/platform/CrossThreadCopier.h
@@ -91,9 +91,14 @@ struct CrossThreadCopier : public CrossThreadCopierBase<T, std::is_arithmetic<T>
 
 // CrossThreadCopier specializations follow.
 template <typename T>
-struct CrossThreadCopier<PassRefPtr<T>> : public CrossThreadCopierPassThrough<PassRefPtr<T>> {
+struct CrossThreadCopier<PassRefPtr<T>> {
     STATIC_ONLY(CrossThreadCopier);
+    typedef T Type;

[Yuta Kitamura, 2016/09/27 01:59:28]

Type should be PassRefPtr<T>, not T. In the original code,
the type parameter of CrossThreadCopierPassThrough was
PassRefPtr<T>, thus Type was PassRefPtr<T>.

(optional: you can also use "using Type = PassRefPtr<T>;"
syntax.)

     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>> {