[proxies] allow duplicate keys for [[OwnPropertyKeys]] trap.

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index 5c0459eb1b0aa11bfc6c6125bac53db27aa2cdae..e83fe4d1df18d4bd81262f20281a451f5b2fed64 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -567,7 +567,8 @@ BasicJsonStringifier::Result BasicJsonStringifier::SerializeJSObject(
     Handle<FixedArray> contents;
     ASSIGN_RETURN_ON_EXCEPTION_VALUE(
         isolate_, contents,
-        JSReceiver::GetKeys(object, JSReceiver::OWN_ONLY, ENUMERABLE_STRINGS),
+        JSReceiver::GetKeys(object, KeyCollectionType::OWN_ONLY,
+                            ENUMERABLE_STRINGS),
         EXCEPTION);
 
     for (int i = 0; i < contents->length(); i++) {