[PaymentRequest] Replace '' with 'unknown' in PaymentComplete.

third_party/WebKit/Source/modules/payments/PaymentResponse.idl

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // https://w3c.github.io/browser-payment-api/#idl-def-paymentcomplete
 
 enum PaymentComplete {
     "success",
     "fail",
-    ""
+    "unknown"
 };
 
 // https://w3c.github.io/browser-payment-api/#paymentresponse-interface
 
 [
     RuntimeEnabled=PaymentRequest,
 ] interface PaymentResponse {
     serializer = {attribute};
 
     readonly attribute DOMString methodName;
     [RuntimeEnabled=PaymentRequestPayerName] readonly attribute DOMString? payerName;
     readonly attribute DOMString? payerEmail;
     readonly attribute DOMString? payerPhone;
     [CallWith=ScriptState, RaisesException] readonly attribute object details;
     readonly attribute PaymentAddress? shippingAddress;
     readonly attribute DOMString? shippingOption;
 
-    [CallWith=ScriptState] Promise<void> complete(optional PaymentComplete paymentResult = "");
+    [CallWith=ScriptState] Promise<void> complete(optional PaymentComplete paymentResult = "unknown");
 };