Verify behavior of PaymentRequest.show() method, part 1.

components/payments/content/payment_request.cc

Index: components/payments/content/payment_request.cc
diff --git a/components/payments/content/payment_request.cc b/components/payments/content/payment_request.cc
index 68da7ff714a86963414c53c99fe4d04bb5fcbb54..9dbe1c5bdba9fb27e3e64c85d65b36830c113d9e 100644
--- a/components/payments/content/payment_request.cc
+++ b/components/payments/content/payment_request.cc
@@ -99,6 +99,14 @@ void PaymentRequest::Show() {
     return;
   }
 
+  // A tab can display only one PaymentRequest UI at a time.
+  if (!manager_->CanShow(this)) {
+    LOG(ERROR) << "A PaymentRequest UI is already showing";
+    client_->OnError(mojom::PaymentErrorReason::USER_CANCEL);
+    OnConnectionTerminated();
+    return;
+  }
+
   if (!state_->AreRequestedMethodsSupported()) {
     client_->OnError(mojom::PaymentErrorReason::NOT_SUPPORTED);
     if (observer_for_testing_)