Can C++ code be called synchronously? Validation errors in constructor should throw, which is synchronous. If C++ cannot be called synchronously, then JavaScript should be the first line of defense against invalid data.

In the absence of "basic-card" method name, the code should perform these two operations:
1) Resolve any caneMakePayment() promise with "false".
2) Reject any show() promise with "NotSupportedError".

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3f2fd127b3f597abbcdeece791353e2c5161df85

commit 3f2fd127b3f597abbcdeece791353e2c5161df85
Author: Mohamad Ahmadi <mahmadi@chromium.org>
Date: Wed Oct 11 19:55:58 2017

[PR] Improved JS validation of payment methods and shipping options

Make the following test cases fully pass:
https://w3c-test.org/payment-request/payment-request-constructor.https.html
https://w3c-test.org/payment-request/payment-request-constructor-crash.https.html
https://w3c-test.org/payment-request/payment-request-ctor-currency-code-checks.https.html
https://w3c-test.org/payment-request/payment-request-ctor-pmi-handling.https.html
https://w3c-test.org/payment-request/payment-request-show-method.https.html
https://w3c-test.org/payment-request/payment-request-show-method.https.html

Bug:  709036 
Change-Id: Ib9e19bba90893588385c0d11a552adb083f34910
Reviewed-on: https://chromium-review.googlesource.com/713577
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508077}
[modify] https://crrev.com/3f2fd127b3f597abbcdeece791353e2c5161df85/ios/chrome/browser/payments/payment_request.mm
[modify] https://crrev.com/3f2fd127b3f597abbcdeece791353e2c5161df85/ios/chrome/browser/web/resources/payment_request.js