[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..818077ccf6cfcdd09c0d3fd2c097650d50f0e60b 100644
--- a/components/payments/core/payment_instrument.h
+++ b/components/payments/core/payment_instrument.h
@@ -37,8 +37,10 @@ class 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;