PasswordManager.FillingAssistance UMA metric implementation.

This metrics tries to understand how good is Chrome Password Manager
in helping the user to sign in. It's recorded only on submissions that
are considered to be successful.

This CL contains:
 1.Implementation the metric calculation in PasswordFormManagerRecorder
 2.Propagating all necessary information for calculation.

Bug: 918846
Change-Id: I53d71a2dcad0c33480157d69e275047138d2df0f
Reviewed-on: https://chromium-review.googlesource.com/c/1400691
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621939}
13 files changed