[Autofill Assistant] Adds histogram for on boarding.

We record the basic user interactions that can happen in
regard on boarding (shown, not_shown, accepted, cancelled).

Bug: 806868
Bug: b/133124492
Change-Id: I84820da9859ee14446db8494fdf8a55ca3004dbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625112
Commit-Queue: Lukasz Suder <lsuder@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663559}