I'll take this issue. The spec work seems finished.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/571755af768f05ca8a7b9f11654a62751df8a452

commit 571755af768f05ca8a7b9f11654a62751df8a452
Author: Jinho Bang <jinho.bang@samsung.com>
Date: Tue Oct 02 00:34:14 2018

PaymentRequest: Implement skeleton code for PaymentMethodChange event

The event handler is triggered when the user changes payment method.
This CL only defines the event in Blink side. A follow-up CL will make
the event fire when user changes payment method on UI.

This feature is still behind runtime flag(PaymentMethodChange).

Intent to implement:
  https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/e8Zrmy52r9w

Related spec link:
  https://w3c.github.io/payment-request/#paymentmethodchangeevent-interface

Bug: 824760
Change-Id: Ica77b46cbeeee78184a2bd66ce2198aae39b0c7a
Reviewed-on: https://chromium-review.googlesource.com/1226774
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Jinho Bang <jinho.bang@samsung.com>
Cr-Commit-Position: refs/heads/master@{#595664}
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/android_webview/tools/system_webview_shell/test/data/webexposed/not-webview-exposed.txt
[delete] https://crrev.com/542d7fd59b14726593c015369ef35a7663a934d5/third_party/WebKit/LayoutTests/external/wpt/payment-request/PaymentMethodChangeEvent/methodDetails-attribute.https-expected.txt
[delete] https://crrev.com/542d7fd59b14726593c015369ef35a7663a934d5/third_party/WebKit/LayoutTests/external/wpt/payment-request/PaymentMethodChangeEvent/methodName-attribute.https-expected.txt
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/WebKit/LayoutTests/external/wpt/payment-request/idlharness.https.window-expected.txt
[delete] https://crrev.com/542d7fd59b14726593c015369ef35a7663a934d5/third_party/WebKit/LayoutTests/external/wpt/payment-request/onpaymentmenthodchange-attribute.https-expected.txt
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/core/events/event_type_names.json5
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/modules_idl_files.gni
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/payments/BUILD.gn
[add] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/payments/payment_method_change_event.cc
[add] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/payments/payment_method_change_event.h
[add] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/payments/payment_method_change_event.idl
[add] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/payments/payment_method_change_event_init.idl
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/payments/payment_request.h
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/payments/payment_request.idl
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/modules/payments/payment_request_update_event.h
[modify] https://crrev.com/571755af768f05ca8a7b9f11654a62751df8a452/third_party/blink/renderer/platform/runtime_enabled_features.json5