SAML - Add feature flag and command line switches

Add a feature `kEnableSamlReauthOnLockscreen` that will gate the
launch of SAML reauthentication on the lock screen. Previously,
the system only relied on the value of the pref
`kSamlLockScreenReauthenticationEnabled`.

Add two command line switches that will assist on testing this feature:

- kSamlLockScreenReauthenticationEnabledOverrideForTests,
  forces the pref value of `kSamlLockScreenReauthenticationEnabled`
  to true.

- kOfflineSignInTimeLimitInSecondsOverrideForTests,
  overrides the prefs controlling the offline time limit
  (kSAMLOfflineSigninTimeLimit & kGaiaOfflineSigninTimeLimitDays)
  with a value from the command line.

Bug: 1102942
Change-Id: I16760924385da944e78dd34d198555a144bd759a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2691218
Commit-Queue: Renato Silva <rrsilva@google.com>
Auto-Submit: Renato Silva <rrsilva@google.com>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#857701}