Make ContentDecryptionModuleResult cross-thread destructible.

Source/platform/heap/Handle.h

Index: Source/platform/heap/Handle.h
diff --git a/Source/platform/heap/Handle.h b/Source/platform/heap/Handle.h
index dc153a8d64edba0c812ef404f143e2a8f39f8f19..31062d062584a457ea66ac4deb2f99d59f6f7042 100644
--- a/Source/platform/heap/Handle.h
+++ b/Source/platform/heap/Handle.h
@@ -454,6 +454,11 @@ private:
     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.