Add a SigninAllowed policy.

chrome/browser/signin/signin_tracker.cc

Index: chrome/browser/signin/signin_tracker.cc
diff --git a/chrome/browser/signin/signin_tracker.cc b/chrome/browser/signin/signin_tracker.cc
index 689717b8a6222eb7a8c0b47d6858fc1be4681ba6..deeb222101485443979c4b409fcdca8e7b7dd76e 100644
--- a/chrome/browser/signin/signin_tracker.cc
+++ b/chrome/browser/signin/signin_tracker.cc
@@ -134,7 +134,8 @@ void SigninTracker::HandleServiceStateChange() {
   }
 
   SigninManager* signin = SigninManagerFactory::GetForProfile(profile_);
-  if (signin->GetAuthenticatedUsername().empty()) {
+  if (signin->GetAuthenticatedUsername().empty() ||
+      !signin->IsSigninAllowed()) {

[sail, 2013/02/08 20:18:57]

This code doesn't make any sense. If sign in is now allowed then why would you
want to trigger a signin failure?

[Andrew T Wilson (Slow), 2013/02/08 20:33:11]

Because signin isn't allowed, so signin should fail?

In any case, we could probably skip this check since SigninManager will
SignOut() if SigninAllowed transitions to true during the signin process.

[Adrian Kuegel, 2013/02/11 16:47:30]

IsSigninAllowed returns true if signin is allowed. So here I check if signin is
not allowed, and if it is not allowed, a signin failure is triggered. This
should cover the case that the policy currently doesn't allow signin, but
somehow a user still was able to sign in. I am not sure if we could ever end up
in this path, so I removed it now.

     // User is signed out, trigger a signin failure.
     state_ = WAITING_FOR_GAIA_VALIDATION;
     observer_->SigninFailed(
@@ -146,8 +147,9 @@ void SigninTracker::HandleServiceStateChange() {
   // Long term, we should separate out service auth failures from the signin
   // process, but for the current UI flow we'll validate service signin status
   // also.
-  ProfileSyncService* service = profile_->IsSyncAccessible() ?
-      ProfileSyncServiceFactory::GetForProfile(profile_) : NULL;
+  ProfileSyncService* service =

[sail, 2013/02/08 20:18:57]

Why this change?

[Adrian Kuegel, 2013/02/11 16:47:30]

Sorry, that was a relict from an earlier version of this changelist, and I
forgot to change it back.

+      profile_->IsSyncAccessible() ?
+          ProfileSyncServiceFactory::GetForProfile(profile_) : NULL;
   if (service && service->waiting_for_auth()) {
     // Still waiting for an auth token to come in so stay in the INITIALIZING
     // state (we do this to avoid triggering an early signin error in the case