[Web Payment][DevTools] Log a change in payment method.

When a payment handler sends a "payment method change" event to the
merchant, the developer tools can display this event and the
corresponding price update from the merchant.

Bug: 980249
Change-Id: Ia3f16215cadf54d0a3f44b6b5f471839ca498f13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742207
Reviewed-by: Danyao Wang <danyao@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685529}
23 files changed