Make ContentDecryptionModuleResult cross-thread destructible.

Source/platform/heap/Handle.h

 /*
  * Copyright (C) 2014 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 436 matching lines @@
 
     String m_tracingName;
 #else
     inline void recordBacktrace() const { }
 #endif
     // m_raw is accessed most, so put it at the first field.
     T* m_raw;
     PersistentNode* m_persistentNode;
 };
 
+// A class T having a public CROSS_THREAD_ACCESSIBLE() declaration makes
+// WebPrivatePtr<T> refer to objects of type T using a CrossThreadPersistent<T>.
+// This to allow WebPrivatePtr<T> objects to be destructed on another thread.
+#define CROSS_THREAD_ACCESSIBLE() typedef int IsCrossThreadAccessibleMarker
+
 // PersistentNode must be the left-most class to let the
 // visitor->trace(static_cast<Collection*>(this)) trace the correct position.
 // FIXME: derive affinity based on the collection.
 template<typename Collection>
 class PersistentHeapCollectionBase : public Collection {
     // We overload the various new and delete operators with using the WTF DefaultAllocator to ensure persistent
     // heap collections are always allocated off-heap. This allows persistent collections to be used in
     // DEFINE_STATIC_LOCAL et. al.
     WTF_USE_ALLOCATOR(PersistentHeapCollectionBase, WTF::DefaultAllocator);
 public:
@@ skipping 733 matching lines @@
 struct ParamStorageTraits<RawPtr<T>> : public PointerParamStorageTraits<T*, blink::IsGarbageCollectedType<T>::value> {
     static_assert(sizeof(T), "T must be fully defined");
 };
 
 template<typename T>
 PassRefPtr<T> adoptRef(blink::RefCountedGarbageCollected<T>*) = delete;
 
 } // namespace WTF
 
 #endif