The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ccb5393c9cc9cf58397674775f48fedc1650a93a

commit ccb5393c9cc9cf58397674775f48fedc1650a93a
Author: Marc-Antoine Courteau <macourteau@chromium.org>
Date: Thu Oct 05 14:04:25 2017

[Payment Request] Enable the done button only when all fields validate.

Also, behave the same was as on desktop, showing validation error
messages in real-time as fields are edited, once they are no longer
pristine (i.e., the user has entered and exited the field already).

Bug:  768387 
Change-Id: I4d8cf605e0e79422e31ca1fc5eda7a9e4161545a
Reviewed-on: https://chromium-review.googlesource.com/698905
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Marc-Antoine Courteau <macourteau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506717}
[modify] https://crrev.com/ccb5393c9cc9cf58397674775f48fedc1650a93a/ios/chrome/browser/ui/payments/payment_request_edit_view_controller.mm
[modify] https://crrev.com/ccb5393c9cc9cf58397674775f48fedc1650a93a/ios/chrome/browser/ui/payments/payment_request_editor_field.h
[modify] https://crrev.com/ccb5393c9cc9cf58397674775f48fedc1650a93a/ios/chrome/browser/ui/payments/payment_request_editor_field.mm

Verified on :

App Version: 63.0.3237.0 canary
Devices : iPhone 7 Plus, iPad Air
iOS Versions: 10.3.3, 11.0

On 'Add Address', 'Add Card'&'Add Contact Info' editors, 'DONE' button will be disabled until all required fields are filled in.

Could test scenarios tapping on 'Incomplete Shipping Address' & 'Incomplete Google Payments cards'editors as the app is crashing (http://crbug.com/767404). 

These cases will be covered once the issue (http://crbug.com/767404) is fixed.

Marking this Bug as 'Verified' for now.