Add ArcEnabled policy implementation

chrome/browser/chromeos/arc/arc_auth_service.h

Index: chrome/browser/chromeos/arc/arc_auth_service.h
diff --git a/chrome/browser/chromeos/arc/arc_auth_service.h b/chrome/browser/chromeos/arc/arc_auth_service.h
index f05fd186e9820d78d4369364ef4e091f8abb07a4..39a4d165406a375c74fc165f25ded43aea85b31a 100644
--- a/chrome/browser/chromeos/arc/arc_auth_service.h
+++ b/chrome/browser/chromeos/arc/arc_auth_service.h
@@ -14,6 +14,7 @@
 #include "components/arc/arc_service.h"
 #include "components/arc/auth/arc_auth_fetcher.h"
 #include "components/arc/common/auth.mojom.h"
+#include "components/prefs/pref_change_registrar.h"
 #include "components/syncable_prefs/pref_service_syncable_observer.h"
 #include "components/syncable_prefs/synced_pref_observer.h"
 #include "google_apis/gaia/gaia_auth_consumer.h"
@@ -170,6 +171,9 @@ class ArcAuthService : public ArcService,
   // communication and shared with Arc OptIn UI platform app.
   content::StoragePartition* storage_partition_ = nullptr;
 
+  // Registrar used to monitor ARC enabled state.
+  PrefChangeRegistrar pref_change_registrar_;
+
   mojo::Binding<AuthHost> binding_;
   base::ThreadChecker thread_checker_;
   State state_ = State::STOPPED;