[api] Prototype WeakRef implementation

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index 7267cbf3a0334cc291331ea8895dd3d4842b6a8e..a91bfb7d5fd14eb697e496c69bfda38b0d49c86d 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -270,6 +270,11 @@ class V8_EXPORT_PRIVATE Factory final {
 
   // Create a promise.
   Handle<JSPromise> NewJSPromise();
+  void InitJSWeakRef(
+    Handle<JSWeakRef> weak_ref,
+    Handle<JSObject> target,
+    Handle<JSFunction> executor,
+    Handle<Object> holdings);
 
   // Create a global (but otherwise uninitialized) context.
   Handle<Context> NewNativeContext();