PaymentHandler: Implement set/get methods in PaymentInstruments (content)

content/browser/payments/payment_app.proto

 // Copyright 2016 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package content;
 
 message PaymentAppManifestProto {
   optional string name = 1;
   optional string icon = 2;
   repeated PaymentAppOptionProto options = 3;
 }
 
 message PaymentAppOptionProto {
   optional string name = 1;
   optional string icon = 2;
   optional string id = 3;
   repeated string enabled_methods = 4;
 }
+
+message PaymentInstrumentProto {
+  optional string name = 1;
+  repeated string enabled_methods = 2;
+  optional string stringified_capabilities = 3;
+}