[Autofill Assistant] Add popup to chips.

This allows chips to show a popup menu when tapped, instead of directly
invoking their callback. For now, this is only intended for trigger
scripts, specifically for the cancel popup.

A screenshot of the popup is in the linked bug.

Bug: b/172585820
Change-Id: Id32c529b7f32a11b2aaef18a9391e708795be38d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521188


Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Luca Hunkeler <hluca@google.com>
Cr-Commit-Position: refs/heads/master@{#824909}