Update SessionStartupPref to support ephemeral Guest profiles.

Session startup pref is updated to support ephemeral Guest profiles.
Ephemeral Guest profiles are not off-the-record and will deprecate OTR
Guest profiles.

This change is behind disabled EnableEphemeralGuestProfilesOnDesktop
flag. Please see go/guest-on-disk for more context.

Bug: 1125474
Change-Id: I786a4f9a3ecccab625936948a6dec798d657ca20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450277
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814171}

chrome/browser/prefs/session_startup_pref.cc

@@ -102,7 +102,7 @@ SessionStartupPref SessionStartupPref::GetStartupPref(const Profile* profile) {
 
   // Guest sessions should not store any state, therefore they should never
   // trigger a restore during startup.
-  return profile->IsGuestSession()
+  return (profile->IsGuestSession() || profile->IsEphemeralGuestProfile())
              ? SessionStartupPref(SessionStartupPref::DEFAULT)
              : GetStartupPref(profile->GetPrefs());
 }