[Autofill Assistant] Handle payment request dismiss.

Before this patch the payment request UI did not properly handle cancel
and back button.

This patch notifies the Autofill Assistant controller to shut down
gracefully with a 'give up' message whenever the payment request UI has
been canceled.

Bug: 806868
Change-Id: Ie3d83bf48a2004f0a6a72763c4b509b3db023ff9
Reviewed-on: https://chromium-review.googlesource.com/c/1326499
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606883}
5 files changed