Revive PasswordManager.PrefilledUsernameFillOutcome histogram

This histogram is main metric in an ongoing experiment
(https://uma.googleplex.com/p/chrome/variations?sid=851f025ba523e095c98306f27c9dd891)
and it was turned off. Revive the metric to make the decision on the
next steps of the experiment.

Bug: 1104971
Change-Id: I696716a83c8b6f96b3aee5268a59157f0eaca7dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4545108


Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Din Nezametdinov <shaikhitdin@google.com>
Cr-Commit-Position: refs/heads/main@{#1146485}