Adds a new checkbox for backup and restore in opt-in flow.

chrome/browser/chromeos/arc/arc_auth_service.cc

Index: chrome/browser/chromeos/arc/arc_auth_service.cc
diff --git a/chrome/browser/chromeos/arc/arc_auth_service.cc b/chrome/browser/chromeos/arc/arc_auth_service.cc
index baf910dbc75001e0d5b5d0f640ed07179366ab75..1f7ec10611a77a418f92f015caaa321be43f87f6 100644
--- a/chrome/browser/chromeos/arc/arc_auth_service.cc
+++ b/chrome/browser/chromeos/arc/arc_auth_service.cc
@@ -40,6 +40,7 @@
 #include "components/arc/arc_bridge_service.h"
 #include "components/policy/core/browser/browser_policy_connector.h"
 #include "components/pref_registry/pref_registry_syncable.h"
+#include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/pref_service.h"
 #include "components/syncable_prefs/pref_service_syncable.h"
 #include "components/user_manager/user.h"
@@ -129,6 +130,11 @@ void ArcAuthService::RegisterProfilePrefs(
 }
 
 // static
+void ArcAuthService::RegisterPrefs(PrefRegistrySimple* registry) {
+  registry->RegisterBooleanPref(prefs::kArcBackupRestoreEnabled, false);

[xiyuan, 2016/06/29 21:36:38]

Think this should be part of user prefs and we should do it with
ArcAuthService::RegisterProfilePrefs above.

+}
+
+// static
 void ArcAuthService::DisableUIForTesting() {
   disable_ui_for_testing = true;
 }