update_engine: Add new rollback policy values

The new values are:
- RollbackAndRestoreIfPossible: always roll back, but try to save and
  restore data if possible. This is rollback_allowed=true.
- RollbackWithRestoreOnly: only roll back if it's possible to save and
  restore data. This is rollback_allowed=false until data save and
  restore is implemented.

BUG=chromium:878736
TEST='cros_run_unit_tests --board=caroline --packages update_engine'

Change-Id: Ifd198610c26d11521b39da8fce5f44dc97609bf5
Reviewed-on: https://chromium-review.googlesource.com/1194229
Commit-Ready: Marton Hunyady <hunyadym@chromium.org>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
diff --git a/update_manager/rollback_prefs.h b/update_manager/rollback_prefs.h
index 1783eb0..07e1834 100644
--- a/update_manager/rollback_prefs.h
+++ b/update_manager/rollback_prefs.h
@@ -30,8 +30,10 @@
   kUnspecified = 0,
   kDisabled = 1,
   kRollbackWithFullPowerwash = 2,
+  kRollbackAndRestoreIfPossible = 3,
+  kRollbackWithRestoreOnly = 4,
   // This value must be the last entry.
-  kMaxValue = 3
+  kMaxValue = 5
 };
 
 }  // namespace chromeos_update_manager