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* Create a new test observer interface PaymentUIObserver
  intended for cross-platform UI event waiters for testing.
* Expand the existing secure_payment_confirmation_browsertests
  to wait until the UI is displayed rather than just waiting for the
  PaymentRequest kAppListReady event. This ensures the controller
  successfully creates the DialogView and calls ShowDialog.

Bug: 1124929
Change-Id: Iacec5998e1b15c4e6def109f2c9a465b0f6abffb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631425
Commit-Queue: Nick Burris <nburris@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844216}