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This patch adds a C++ payment app object that communicates to Android
payment apps. At this time, only Chrome OS has concrete implementation
that connects to a Trusted Web Activity in the Android subsystem.

Design: https://bit.ly/cross-platform-pay-app-factory

Bug: 1061503
Change-Id: Ice5918967493e34cfa5ad4a4ccc25ce2602d204a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255112
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798238}