Print Preview: Show error message if text is typed in Copies section

chrome/browser/resources/print_preview/data/ticket_items/copies.js

 // Copyright (c) 2012 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.
 
 cr.define('print_preview.ticket_items', function() {
   'use strict';
 
   /**
    * Copies ticket item whose value is a {@code string} that indicates how many
    * copies of the document should be printed. The ticket item is backed by a
    * string since the user can textually input the copies value.
    * @param {!print_preview.DestinationStore} destinationStore Destination store
    *     used determine if a destination has the copies capability.
    * @constructor
    * @extends {print_preview.ticket_items.TicketItem}
    */
   function Copies(destinationStore) {
     print_preview.ticket_items.TicketItem.call(
         this, null /*appState*/, null /*field*/, destinationStore);
   };
 
   Copies.prototype = {
     __proto__: print_preview.ticket_items.TicketItem.prototype,
 
     /** @override */
     wouldValueBeValid: function(value) {
+      if (value == '') {
+        return false;
+      }
       if (/[^\d]+/.test(value)) {

[dpapad, 2017/02/24 20:35:35]

Nit (optional): Maybe combine the two conditions.

if (value == '' || /[^\d]+/.test(value)) {
  return false;
}

         return false;

[dpapad, 2017/02/24 20:35:35]

Suggestion for improvement, can be done separately, but I'd like to know WDYT?

Now that we use native numerical input, we can take advantage of its native
validation logic and error reporting. See https://jsfiddle.net/5g682z0g/.
 1) We can remove all the custom validatino logic here and simply check
input.validity.valid
 2) When not valid, we can simply call input.reportValidity() and a native error
message will be displayed.

I think this would be a good improvement (and we would be using the platform
more).

       }
       var copies = parseInt(value, 10);
       if (copies > 999 || copies < 1) {
         return false;
       }
       return true;
     },
 
     /** @override */
     isCapabilityAvailable: function() {
@@ skipping 28 matching lines @@
               dest.capabilities.printer.copies) ||
              null;
     }
   };
 
   // Export
   return {
     Copies: Copies
   };
 });