[Payments] Show what's missing for incomplete payment methods.

components/payments/core/autofill_payment_instrument.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_PAYMENTS_CORE_AUTOFILL_PAYMENT_INSTRUMENT_H_
 #define COMPONENTS_PAYMENTS_CORE_AUTOFILL_PAYMENT_INSTRUMENT_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 23 matching lines @@
       const std::string& method_name,
       const autofill::CreditCard& card,
       const std::vector<autofill::AutofillProfile*>& billing_profiles,
       const std::string& app_locale,
       PaymentRequestDelegate* payment_request_delegate);
   ~AutofillPaymentInstrument() override;
 
   // PaymentInstrument:
   void InvokePaymentApp(PaymentInstrument::Delegate* delegate) override;
   bool IsCompleteForPayment() override;
+  base::string16 GetMissingInfoLabel() override;
   bool IsValidForCanMakePayment() override;
 
   // autofill::payments::FullCardRequest::ResultDelegate:
   void OnFullCardRequestSucceeded(const autofill::CreditCard& card,
                                   const base::string16& cvc) override;
   void OnFullCardRequestFailed() override;
 
   autofill::CreditCard* credit_card() { return &credit_card_; }
 
  private:
   // A copy of the card is owned by this object.
   autofill::CreditCard credit_card_;
   // Not owned by this object, should outlive this.
   const std::vector<autofill::AutofillProfile*>& billing_profiles_;
 
   const std::string app_locale_;
 
   PaymentInstrument::Delegate* delegate_;
   PaymentRequestDelegate* payment_request_delegate_;
 
   base::WeakPtrFactory<AutofillPaymentInstrument> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(AutofillPaymentInstrument);
 };
 
 }  // namespace payments
 
 #endif  // COMPONENTS_PAYMENTS_CORE_AUTOFILL_PAYMENT_INSTRUMENT_H_