[Payments] Display the shipping option error alongside the address

components/payments/content/payment_request_spec.h

Index: components/payments/content/payment_request_spec.h
diff --git a/components/payments/content/payment_request_spec.h b/components/payments/content/payment_request_spec.h
index 905cfb5fcc6533e7ce680f7f870bcf6b4376bb2c..30cad02cf7430e28bf112b161c5656fae8e7556a 100644
--- a/components/payments/content/payment_request_spec.h
+++ b/components/payments/content/payment_request_spec.h
@@ -94,6 +94,11 @@ class PaymentRequestSpec : public PaymentOptionsProvider {
   mojom::PaymentShippingOption* selected_shipping_option() const {
     return selected_shipping_option_;
   }
+  // if |selected_shipping_option()| is nullptr, this may contain a non-empty
+  // error returned by the merchant.
+  const std::string& selected_shipping_option_error() const {
+    return selected_shipping_option_error_;
+  }
 
   const mojom::PaymentDetails& details() const { return *details_.get(); }
 
@@ -132,6 +137,7 @@ class PaymentRequestSpec : public PaymentOptionsProvider {
   const std::string app_locale_;
   // The currently shipping option as specified by the merchant.
   mojom::PaymentShippingOption* selected_shipping_option_;
+  std::string selected_shipping_option_error_;
 
   std::unique_ptr<CurrencyFormatter> currency_formatter_;