Update the shippingType option to not allow invalid strings.

spec list:
https://w3c.github.io/browser-payment-api/#dom-paymentoptions
https://github.com/w3c/browser-payment-api/pull/416

BUG=690928

Review-Url: https://codereview.chromium.org/2685343002
Cr-Commit-Position: refs/heads/master@{#450379}
2 files changed