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

components/payments/core/payment_instrument.h

Index: components/payments/core/payment_instrument.h
diff --git a/components/payments/core/payment_instrument.h b/components/payments/core/payment_instrument.h
index 5d70847f4f273358ce06978c94fb60caf22e148d..608235be4eaef5f8685fdbca2c8184d34e2557b5 100644
--- a/components/payments/core/payment_instrument.h
+++ b/components/payments/core/payment_instrument.h
@@ -39,6 +39,9 @@ class PaymentInstrument {
   // Returns whether the instrument is complete to be used as a payment method
   // without further editing.
   virtual bool IsCompleteForPayment() = 0;
+  // Returns a message to indicate to the user what's missing for the instrument
+  // to be complete for payment.
+  virtual base::string16 GetMissingInfoLabel() = 0;
   // Returns whether the instrument is valid for the purposes of responding to
   // canMakePayment.
   virtual bool IsValidForCanMakePayment() = 0;