[Passwords] Introduce PasswordManager.FillingAssistance suffixes

This change introduces a histogram suffixes for
PasswordManager.FillingAssistance that detail whether the form was
filled on a secure or insecure origin. This is useful for experiments
that only target a subset of origins, for example the ongoing HTTP
FOAS experiment.

TBR=jdoerrie@chromium.org

(cherry picked from commit 5a39502e15ac0f3dd880590e423461eb72802d2d)

Bug: 918846, 914765
Change-Id: I6ae96701bbbbd2b364e3e52c4f1ec35e163f499f
Reviewed-on: https://chromium-review.googlesource.com/c/1446155
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#628062}
Reviewed-on: https://chromium-review.googlesource.com/c/1449576
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#143}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
3 files changed