Create Autofill.FillingProduct variants

This CL defines the Autofill.FillingProduct variants and uses it for all
the histograms that are currently composed of a filling product. This
will improve consistency across those histograms.

This CL also adds lint checks to the new variants and to the
`FillingProductToString` method to make sure that those two stay in
sync.

Note: Some FillingProducts are currently not considered for some of the
modified metrics, and will now be with this change. However, there's no
need to create V2s of these histograms since we're adding new histograms
and not modifying the way existing histograms are logged (this was
confirmed by a member of the Chrome Metrics team).

Bug: None
Change-Id: I60b845147faa0c6ec35ed7dc0d9e2a9544e49e31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5839021
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Noémie St-Onge <noemies@google.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1354023}