[Autofill] Add metric to log when local save is successful

We're looking to merge this (ideally temporary) metric into M71.
A problem has been discovered with regards to checkout flows that span
multiple domains, and this will help determine its impact.

Bug: 892299
Change-Id: I79c9acba23eb750aecf0171ee92d0de456c2ce0e
Reviewed-on: https://chromium-review.googlesource.com/c/1315953
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Jared Saul <jsaul@google.com>
Cr-Commit-Position: refs/heads/master@{#605825}