[Autofill Assistant] Added configurable timeout to the JITT prompt.

This is intended to be used in exceptional circumstances only,
specifically when scroll_to_hide does not work. By default, there is no
timeout.

This also fixes a possible race condition where page navigations would
not always reset the NOT_NOW status of temporarily hidden prompts.

Bug: b/192908676
Change-Id: I35ca17e2c175aa33d2050ff20a57584b2966683e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3016645


Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#900958}