[Payment Handler][Desktop] Separate PaymentHandlerHost into its own class

This patch separates the implementation of mojom::PaymentHandlerHost
interface from payments::PaymentRequest into its own class
payments::PaymentHandlerHost, so it can be used from Java in an upcoming
patch. Instead of payments::PaymentRequest being a
mojom::PaymentHandlerHost, payments::PaymentRequest now has a private
member payments::PaymentHandlerHost, which implements the
mojom::PaymentHandlerHost interface.

Bug: 884680
Change-Id: I581bd73243be1302f72973b7ce01b5c397efc537
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604311
Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658644}
5 files changed