https://w3c-test.org/payment-request/payment-request-ctor-pmi-handling.https.html

Sounds like a fun one. Did we not already do validation renderer side for PMI? 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0beaddc405726742b5545de252e6bf0a2d43edba

commit 0beaddc405726742b5545de252e6bf0a2d43edba
Author: Rouslan Solomakhin <rouslan@chromium.org>
Date: Wed Sep 20 16:04:10 2017

[Payments] Validate payment method format.

https://w3c.github.io/payment-request/#constructor

"For each paymentMethod of methodData:
 Run the steps to validate a payment method identifier with
 paymentMethod.supportedMethods. If it returns false, then throw a
 RangeError exception and terminate this algorithm."

https://www.w3.org/TR/payment-method-id/#dfn-validate-a-payment-method-identifier

"The steps to validate a payment method identifier with a string pmi are
 given by the following algorithm. It returns true if the pmi is valid.

 1. Let url be the result of running the basic URL parser with pmi.
 2. If url is failure, validate a standardized payment method identifier
    with pmi and return the result.
 3. Otherwise, validate a URL-based payment method identifier passing
    url and return the result."

Bug:  766805 
Change-Id: I1b7dad94a90bd388751335bd95558ed812326ee3
Reviewed-on: https://chromium-review.googlesource.com/673276
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503164}
[modify] https://crrev.com/0beaddc405726742b5545de252e6bf0a2d43edba/chrome/browser/ui/views/payments/payment_request_browsertest.cc
[delete] https://crrev.com/fa9689106cb62897fd78094e0c36d89a833b49b8/third_party/WebKit/LayoutTests/external/wpt/payment-request/payment-request-ctor-pmi-handling.https-expected.txt
[modify] https://crrev.com/0beaddc405726742b5545de252e6bf0a2d43edba/third_party/WebKit/Source/modules/payments/PaymentRequest.cpp

> Did we not already do validation renderer side for PMI? 

No, our validation was all on the browser side.