digitalgoods:Ensure "payments" feature is enabled to use DG.

Trying to match the restrictions in the Payment Request API spec [1]
(DG isn't spec'd yet).

We require that the "payment" feature policy is enabled. Secure context
is already ensured by the IDL.
Also add some logging of errors so developers can figure out what is
going wrong.

[1]: https://w3c.github.io/payment-request/#dfn-payment-relevant-browsing-context

Bug: 1061503
Change-Id: I2c13663148ad8736a4d9ab3ea124d96038d8e878
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526872Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826681}

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

@@ -23,7 +23,7 @@ void OnCreateDigitalGoodsResponse(
     mojo::PendingRemote<payments::mojom::blink::DigitalGoods> pending_remote) {
   if (code != CreateDigitalGoodsResponseCode::kOk) {
     DCHECK(!pending_remote);
-    DVLOG(1) << "CreateDigitalGoodsResponseCode " << code;
+    VLOG(1) << "CreateDigitalGoodsResponseCode " << code;
     resolver->Resolve(v8::Null(resolver->GetScriptState()->GetIsolate()));
     return;
   }
@@ -53,14 +53,36 @@ ScriptPromise DOMWindowDigitalGoods::GetDigitalGoodsService(
   auto promise = resolver->Promise();
 
   if (payment_method.IsEmpty()) {
+    VLOG(1) << "GetDigitalGoodsService error: Empty payment method.";
+    resolver->Resolve(v8::Null(script_state->GetIsolate()));
+    return promise;
+  }
+
+  if (!script_state->ContextIsValid()) {
+    VLOG(1) << "GetDigitalGoodsService error: Context invalid.";
+    resolver->Resolve(v8::Null(script_state->GetIsolate()));
+    return promise;
+  }
+
+  auto* execution_context = ExecutionContext::From(script_state);
+  DCHECK(execution_context);
+
+  if (execution_context->IsContextDestroyed()) {
+    VLOG(1) << "GetDigitalGoodsService error: Context destroyed.";
+    resolver->Resolve(v8::Null(script_state->GetIsolate()));
+    return promise;
+  }
+
+  if (!execution_context->IsFeatureEnabled(
+          mojom::blink::FeaturePolicyFeature::kPayment)) {
+    VLOG(1) << "GetDigitalGoodsService error: Payments not enabled.";
     resolver->Resolve(v8::Null(script_state->GetIsolate()));
     return promise;
   }
 
   if (!mojo_service_) {
-    ExecutionContext::From(script_state)
-        ->GetBrowserInterfaceBroker()
-        .GetInterface(mojo_service_.BindNewPipeAndPassReceiver());
+    execution_context->GetBrowserInterfaceBroker().GetInterface(
+        mojo_service_.BindNewPipeAndPassReceiver());
   }
 
   mojo_service_->CreateDigitalGoods(