Update Chrome Signin client to cover ephemeral Guest profile.

The "force signin" policy is updated to handle ephemeral Guest profiles
similar to off the record Guest profiles.

Please see go/ephemeral-guest-profiles for more context.

Bug: 1125474
Change-Id: I5527423c3fbc9abe3ac005242b11203d25121066
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577682
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834662}

chrome/browser/signin/chrome_signin_client.cc

@@ -172,8 +172,8 @@ void ChromeSigninClient::PreSignOut(
       signout_source_metric == signin_metrics::SERVER_FORCED_DISABLE ||
       signout_source_metric == signin_metrics::SIGNOUT_PREF_CHANGED;
   if (signin_util::IsForceSigninEnabled() && !profile_->IsSystemProfile() &&
-      !profile_->IsGuestSession() && !profile_->IsSupervised() &&
-      !keep_window_opened) {
+      !profile_->IsGuestSession() && !profile_->IsEphemeralGuestProfile() &&
+      !profile_->IsSupervised() && !keep_window_opened) {
     if (signout_source_metric ==
         signin_metrics::SIGNIN_PREF_CHANGED_DURING_SIGNIN) {
       // SIGNIN_PREF_CHANGED_DURING_SIGNIN will be triggered when