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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/1226774Reviewed-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}