Load the and store rollback version into the correct variable.

Somehow missed this and added unittest to help never have this issue again.
In this CL I also got rid of the powerwash_prefs_ logic in GetPersistedValue
because it was just useless noise in my prev CL that isn't used anymore.

BUG=chromium:285381
TEST=unittests that actually catch error.

Change-Id: I4ed680c5b5c2e37549d58aa0a7c0d35a62623fe7
Reviewed-on: https://chromium-review.googlesource.com/168073
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
diff --git a/payload_state.h b/payload_state.h
index f358bda..ad0f339 100644
--- a/payload_state.h
+++ b/payload_state.h
@@ -114,6 +114,7 @@
   FRIEND_TEST(PayloadStateTest, RebootAfterUpdateFailedMetric);
   FRIEND_TEST(PayloadStateTest, RebootAfterUpdateSucceed);
   FRIEND_TEST(PayloadStateTest, RebootAfterCanceledUpdate);
+  FRIEND_TEST(PayloadStateTest, RollbackVersion);
   FRIEND_TEST(PayloadStateTest, UpdateSuccessWithWipedPrefs);
 
   // Increments the payload attempt number used for metrics.
@@ -181,10 +182,9 @@
   // reset on a new update.
   void ResetDownloadSourcesOnNewUpdate();
 
-  // Returns the persisted value for the given key. It also validates that
-  // the value returned is non-negative. If |across_powerwash| is True,
-  // get the value that will persist across a powerwash.
-  int64_t GetPersistedValue(const std::string& key, bool across_powerwash);
+  // Returns the persisted value from prefs_ for the given key. It also
+  // validates that the value returned is non-negative.
+  int64_t GetPersistedValue(const std::string& key);
 
   // Calculates the response "signature", which is basically a string composed
   // of the subset of the fields in the current response that affect the