Add a histogram for /samlredirect usage on ChromeOS

Gaia endpoint /samlredirect can be used during online sign-in flows to
land directly on a 3P IdP pages. Add a histogram which records when:
(1) we start samlredirect flow based on an SSO profile
(2) we start based on a domain
(3) user clicks the button to switch to the default Google sign-in

This is relevant both for login screen and lock screen online flows, so
there are variations of this histogram for Login and Unlock.

Bug: b:441744001
Change-Id: Ia1b3b4efc9d933f1b9f8702b9680ce600be7a847
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6997954
Commit-Queue: Andrey Davydov <andreydav@google.com>
Reviewed-by: Renato Silva <rrsilva@google.com>
Reviewed-by: Felipe Andrade <fsandrade@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1526826}