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..fbf44c1801bde14191bfd3a46295e3841feebbd7 100644
--- a/third_party/WebKit/Source/platform/CrossThreadCopier.h
+++ b/third_party/WebKit/Source/platform/CrossThreadCopier.h
@@ -94,6 +94,10 @@ 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>> {