Add BackupRollbackController for starting backup/rollback

components/sync_driver/pref_names.cc

Index: components/sync_driver/pref_names.cc
diff --git a/components/sync_driver/pref_names.cc b/components/sync_driver/pref_names.cc
index 7ab338270d196308306ce5a31b91cda1cbafcdf0..c8cafc2c58992af02613ade393206b177852ad48 100644
--- a/components/sync_driver/pref_names.cc
+++ b/components/sync_driver/pref_names.cc
@@ -95,6 +95,9 @@ const char kSyncSessionsGUID[] = "sync.session_sync_guid";
 const char kSyncSpareBootstrapToken[] = "sync.spare_bootstrap_token";
 #endif  // defined(OS_CHROMEOS)
 
+// Stores how many times to try rollback before giving up.
+const char kSyncRemainingRollbackTries[] = "sync.remaining_rollback_tries";
+
 }  // namespace prefs
 
 }  // namespace sync_driver