Form events at UKM level

1) UKM-ifying form events. UMA metrics are heavily used and just getting that to UKM.
Number of aspects are interesting to look at URL level also here. Form event proto for reference - https://cs.chromium.org/chromium/src/components/autofill/core/browser/autofill_metrics.h?l=597
2) Deprecating few redundant methods. Will do more following clean up.
3) Enforcing *timestamp* where ever it makes sense and in a way across all as post processing
pipelines uses it for multiple purposes.
4) Enforced *IsAutofillable" as check to log anything in FormEvents as all form events are only logged
after form is detected. Will separate out logging for more recall improvement purposes for non
IsAutofillable part in a later CL.

Privacy review approved - https://docs.google.com/document/d/1-ukZzPZjjl6LO5bOpEzW_eUimzWTaL1Wn94S3BlpGuk/edit

Followup
-> Clean up of Autofill user happiness metrics as that is not useful that much.
-> Few more UKM metrics (related to recall)

Change-Id: I1b088cd0a7e496ae56e0ea6a240f776e05eca0ec
Reviewed-on: https://chromium-review.googlesource.com/c/1400262
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Jared Saul <jsaul@google.com>
Commit-Queue: Lakshmi Kumar Dabbiru <dlkumar@google.com>
Cr-Commit-Position: refs/heads/master@{#634985}
12 files changed