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

components/payments/core/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_PAYMENT_INSTRUMENT_H_
 #define COMPONENTS_PAYMENTS_CORE_PAYMENT_INSTRUMENT_H_
 
 #include <set>
 #include <string>
 
@@ skipping 19 matching lines @@
         const std::string& stringified_details) = 0;
 
     virtual void OnInstrumentDetailsError() = 0;
   };
 
   virtual ~PaymentInstrument();
 
   // Will call into the |delegate| (can't be null) on success or error.
   virtual void InvokePaymentApp(Delegate* delegate) = 0;
   // Returns whether the instrument is complete to be used as a payment method
-  // without further editing.
-  virtual bool IsCompleteForPayment() = 0;
+  // without further editing. If it's not complete, returns false and fills
+  // |missing_info| with the message. NOTE: callers who do not care about the
+  // missing info message can pass nullptr.
+  virtual bool IsCompleteForPayment(base::string16* missing_info) = 0;
   // Returns whether the instrument is valid for the purposes of responding to
   // canMakePayment.
   virtual bool IsValidForCanMakePayment() = 0;
 
   const std::string& method_name() const { return method_name_; }
   const base::string16& label() const { return label_; }
   const base::string16& sublabel() const { return sublabel_; }
   int icon_resource_id() const { return icon_resource_id_; }
   Type type() { return type_; }
 
@@ skipping 10 matching lines @@
   const base::string16 sublabel_;
   int icon_resource_id_;
   Type type_;
 
   DISALLOW_COPY_AND_ASSIGN(PaymentInstrument);
 };
 
 }  // namespace payments
 
 #endif  // COMPONENTS_PAYMENTS_CORE_PAYMENT_INSTRUMENT_H_