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/boxed_value.cc b/update_manager/boxed_value.cc
index 971e9b7..c987110 100644
--- a/update_manager/boxed_value.cc
+++ b/update_manager/boxed_value.cc
@@ -146,6 +146,10 @@
       return "Disabled";
     case RollbackToTargetVersion::kRollbackWithFullPowerwash:
       return "Rollback with full powerwash";
+    case RollbackToTargetVersion::kRollbackAndRestoreIfPossible:
+      return "Rollback and restore if possible";
+    case RollbackToTargetVersion::kRollbackWithRestoreOnly:
+      return "Rollback with restore only";
     case RollbackToTargetVersion::kMaxValue:
       NOTREACHED();
       return "Max value";