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

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

Index: chrome/browser/resources/print_preview/data/ticket_items/copies.js
diff --git a/chrome/browser/resources/print_preview/data/ticket_items/copies.js b/chrome/browser/resources/print_preview/data/ticket_items/copies.js
index 91e6acf475ddeafc719e53535df5c29632628fcb..2d5175094fe2546e2ffdee9c3e0502c912c94750 100644
--- a/chrome/browser/resources/print_preview/data/ticket_items/copies.js
+++ b/chrome/browser/resources/print_preview/data/ticket_items/copies.js
@@ -24,6 +24,9 @@ cr.define('print_preview.ticket_items', function() {
 
     /** @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).

       }