update_engine: Place enterprise rollback save marker file

- There are now two types of rollback. One that does a full powerwash
  and one that save some system state.
- When an enterprise rollback powerwash is scheduled, place a marker
  file to tell the shutdown process to save data before rebooting.
- This lets rollback preserve additional data over a powerwash that
  can be restored later.
- Change a few places that were using is_rollback to save_rollback_data
  to be explicit.

BUG=chromium:955463
TEST=unittests

Change-Id: I9f18319e711e425a6e712dd319e03bcc6ddd0a1b
Reviewed-on: https://chromium-review.googlesource.com/1414030
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/payload_consumer/install_plan.h b/payload_consumer/install_plan.h
index ede36b3..17cefd8 100644
--- a/payload_consumer/install_plan.h
+++ b/payload_consumer/install_plan.h
@@ -146,6 +146,9 @@
   // True if this update is a rollback.
   bool is_rollback{false};
 
+  // True if this rollback should preserve some system data.
+  bool rollback_data_save_requested{false};
+
   // True if the update should write verity.
   // False otherwise.
   bool write_verity{true};