Hi Sanjoy, I would like to do this as part of actually sending id to native app:
https://codereview.chromium.org/2770193003/

Is that ok with you?

Please take it up.

Thanks Sanjoy!
Got my patch up here:
https://codereview.chromium.org/2770193003/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf

commit c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf
Author: rob.buis <rob.buis@samsung.com>
Date: Fri Apr 28 02:53:35 2017

Implement request id in PaymentDetailsInit

Implement request id in PaymentDetailsInit as well as id
getter in PaymentRequest.

In order for this functionality to make sense we pass the
id to the payment app and make sure to set it in the
PaymentResponse through the requestId attribute.

Added PaymentRequestIdTest as a payment integration test for verifying
the PaymentResponse contains the free-form identifier specified in
PaymentDetailsInit.

Added PaymentRequestTest.DetailsIdIsSet unit test to verify that
PaymentDetailsInit.id is reflected in PaymentRequest.id.

Added external/wpt/payment-request/payment-request-id.https.html to test that
PaymentDetailsInit.id is reflected in PaymentRequest.id and that not providing
an id generates a UUID.

Added manual test external/wpt/payment-request/payment-request-response-id.html
for verifying the PaymentResponse contains the free-form identifier specified
in PaymentDetailsInit.

Intent to Implement and Ship thread:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/WaHAg3wh7jw/vWAqxSnOCgAJ

BUG= 701254 
TEST=PaymentRequestIdTest, DetailsIdIsSet

Review-Url: https://codereview.chromium.org/2770193003
Cr-Commit-Position: refs/heads/master@{#467854}

[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/android/java/src/org/chromium/chrome/browser/payments/AndroidPaymentApp.java
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/android/java/src/org/chromium/chrome/browser/payments/AutofillPaymentInstrument.java
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/android/java/src/org/chromium/chrome/browser/payments/PaymentInstrument.java
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/android/java/src/org/chromium/chrome/browser/payments/PaymentRequestImpl.java
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/android/java/src/org/chromium/chrome/browser/payments/ServiceWorkerPaymentInstrument.java
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/android/java_sources.gni
[add] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/android/javatests/src/org/chromium/chrome/browser/payments/PaymentRequestIdTest.java
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/android/javatests/src/org/chromium/chrome/browser/payments/PaymentRequestTestBase.java
[add] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/test/data/payments/payment_request_id.js
[add] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/chrome/test/data/payments/payment_request_id_test.html
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/components/payments/mojom/payment_request.mojom
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/LayoutTests/external/wpt/payment-request/interfaces.https-expected.txt
[add] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/LayoutTests/external/wpt/payment-request/payment-request-id.https.html
[add] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/LayoutTests/external/wpt/payment-request/payment-request-response-id.html
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/LayoutTests/virtual/service-worker-navigation-preload-disabled/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentDetailsInit.idl
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentRequest.cpp
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentRequest.h
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentRequest.idl
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentRequestTest.cpp
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentResponse.cpp
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentResponse.h
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentResponse.idl
[modify] https://crrev.com/c538713fa88d7fb0a831fbaa3b7dedafb9b5cdbf/third_party/WebKit/Source/modules/payments/PaymentResponseTest.cpp