[Autofill Assistant] Support highlighted button in PromptAction.

Before this change, PromptAction only supported normal chips. This
change adds support for displaying choice as scripts in buttons with
highlights.

The way this is done reproduces exactly the way buttons are configured
for scripts, sharing the same logic and code. The idea is that it would
help keep the two consistent.

Bug: 806868
Change-Id: I82b3a49b1339e4aecd1c3dffabb826022fbac1e6
Reviewed-on: https://chromium-review.googlesource.com/c/1348050
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610648}
6 files changed