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This adds the content setting page for the new PaymentHandler API and
the UI plumbing. The page is chrome://settings/content/paymentHandler.
This is behind the kServiceWorkerPaymentApps feature flag.

Bug: 665949
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ie388940363c7efda57e66ba16dbf6edcf0f5a31c
Reviewed-on: https://chromium-review.googlesource.com/934741
Commit-Queue: Jinho Bang <jinho.bang@samsung.com>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544102}