digitalgoods:Make GetDigitalGoodsService return null not undefined.

This matches the explainer and makes it more explicit that the service
was not available intentionally.

Bug: 1131747
Change-Id: I9afca606f000d969b05b1e5cff9f95b124617b83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2497944Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821629}

third_party/blink/renderer/modules/payments/goods/dom_window_digital_goods.cc

@@ -26,7 +26,7 @@ void OnCreateDigitalGoodsResponse(
   if (code != CreateDigitalGoodsResponseCode::kOk) {
     DCHECK(!pending_remote);
     DVLOG(1) << "CreateDigitalGoodsResponseCode " << code;
-    resolver->Resolve();
+    resolver->Resolve(v8::Null(resolver->GetScriptState()->GetIsolate()));
     return;
   }
   DCHECK(pending_remote);
@@ -55,11 +55,11 @@ ScriptPromise DOMWindowDigitalGoods::GetDigitalGoodsService(
   auto promise = resolver->Promise();
 
   if (payment_method.IsEmpty()) {
-    resolver->Resolve();
+    resolver->Resolve(v8::Null(script_state->GetIsolate()));
     return promise;
   }
   if (payment_method != known_payment_method_) {
-    resolver->Resolve();
+    resolver->Resolve(v8::Null(script_state->GetIsolate()));
     return promise;
   }
 

third_party/blink/web_tests/digital-goods/digital-goods-interface.html

@@ -14,16 +14,20 @@
 <script>
 'use strict';
 
-digital_goods_test(
-    async service => assert_not_equals(service, undefined),
-    {title: 'Digital goods service exists.'});
-
-digital_goods_test(
-    async service => assert_equals(service, undefined),
-    {
-      paymentMethod: 'https://not.known/payment/method',
-      title: 'Digital goods service null for unknown payment method.'
-    });
+digital_goods_test(async service => {
+  assert_not_equals(service, undefined);
+}, {title: 'Digital goods service exists.'});
+
+digital_goods_test(async service => {
+  assert_not_equals(service, null);
+}, {title: 'Digital goods service is available.'});
+
+digital_goods_test(async service => {
+  assert_equals(service, null);
+}, {
+  paymentMethod: 'https://not.known/payment/method',
+  title: 'Digital goods service null for unknown payment method.'
+});
 
 digital_goods_test(async service => {
   const response = await service.getDetails(['id2']);