[Web Payments] Implement the credit card selection UI functionality.

components/payments/payment_request.h

Index: components/payments/payment_request.h
diff --git a/components/payments/payment_request.h b/components/payments/payment_request.h
index 0207c6b1b705398b3bc83964b2227d12ac3ff805..551d8ffec00d98c8a43cdf033bf513b6ecda1b24 100644
--- a/components/payments/payment_request.h
+++ b/components/payments/payment_request.h
@@ -100,6 +100,10 @@ class PaymentRequest : payments::mojom::PaymentRequest {
   // card.
   autofill::CreditCard* selected_credit_card() { return selected_credit_card_; }
 
+  void set_selected_credit_card(autofill::CreditCard* credit_card) {
+    selected_credit_card_ = credit_card;
+  }
+
   autofill::PersonalDataManager* personal_data_manager() {
     return delegate_->GetPersonalDataManager();
   }