[Autofill Assistant] Allow override of AA PR confirm button text.

This is the client change to allow scripters to override the text of the Autofill-Assistant payment request button.

Bug: b/131363578
Change-Id: I23990038b6636507fd8df3654aacddd9f5d43901
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1603348
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658687}