This is necessary to have a '+' image trailing for the Add [...] items, instead of the '>' disclosure indicator.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7e6d99ed0c329815480dbde0cd29661b7d5f4872

commit 7e6d99ed0c329815480dbde0cd29661b7d5f4872
Author: Marc-Antoine Courteau <macourteau@chromium.org>
Date: Mon Oct 16 17:46:00 2017

[Payment Request] Adds support for a trailing image in PaymentsTextItem.

Bug:  774499 
Change-Id: I6625909272de2ce0c975d8733cb0c981ac2d5d09
Reviewed-on: https://chromium-review.googlesource.com/719477
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Marc-Antoine Courteau <macourteau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509094}
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/billing_address_selection_mediator.mm
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/cells/payments_text_item.h
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/cells/payments_text_item.mm
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/cells/payments_text_item_unittest.mm
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/contact_info_selection_mediator.mm
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/payment_method_selection_mediator.mm
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/payment_request_edit_view_controller.mm
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/shipping_address_selection_mediator.mm
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/payments/shipping_option_selection_mediator.mm
[modify] https://crrev.com/7e6d99ed0c329815480dbde0cd29661b7d5f4872/ios/chrome/browser/ui/settings/material_cell_catalog_view_controller.mm

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9d4b57843868e38e1e8f15f44450486b92aa7e79

commit 9d4b57843868e38e1e8f15f44450486b92aa7e79
Author: Marc-Antoine Courteau <macourteau@chromium.org>
Date: Mon Oct 16 18:49:42 2017

[Payment Request] Uses the '+' icon for Add items.

Screenshot: https://imgur.com/a/B8M4U

Bug:  774499 
Change-Id: I4ce23aa677f45d41e074f3b63715a0f8ea877531
Reviewed-on: https://chromium-review.googlesource.com/719723
Commit-Queue: Marc-Antoine Courteau <macourteau@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509120}
[modify] https://crrev.com/9d4b57843868e38e1e8f15f44450486b92aa7e79/ios/chrome/browser/ui/payments/BUILD.gn
[modify] https://crrev.com/9d4b57843868e38e1e8f15f44450486b92aa7e79/ios/chrome/browser/ui/payments/payment_request_mediator.mm
[modify] https://crrev.com/9d4b57843868e38e1e8f15f44450486b92aa7e79/ios/chrome/browser/ui/payments/payment_request_mediator_unittest.mm
[add] https://crrev.com/9d4b57843868e38e1e8f15f44450486b92aa7e79/ios/chrome/browser/ui/payments/resources/BUILD.gn
[add] https://crrev.com/9d4b57843868e38e1e8f15f44450486b92aa7e79/ios/chrome/browser/ui/payments/resources/ic_add.imageset/Contents.json
[add] https://crrev.com/9d4b57843868e38e1e8f15f44450486b92aa7e79/ios/chrome/browser/ui/payments/resources/ic_add.imageset/ic_add@2x.png
[add] https://crrev.com/9d4b57843868e38e1e8f15f44450486b92aa7e79/ios/chrome/browser/ui/payments/resources/ic_add.imageset/ic_add@3x.png