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This CL is to add a test for Expandable Payment Handler UI.

Main Change:
* add 1 test
* add maxpay.com payment app

Flag enabled: ScrollToExpandPaymentHandler. With this flag, Payment
Request will open the Expandable PH UI instead of the CCT PH UI.

Test Steps:
1. install payment app - maxpay.com
2. launch the app
3. when the app is on load, invoke a callback to respond "success" to
the PaymentRequest.

Bug: 1042892

Change-Id: I8a64a6f892eff8c9ae3c6c98acd948938b7eb173
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980187
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732937}