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_unittest.cc b/payload_state_unittest.cc
index 613940e..d8032fe 100644
--- a/payload_state_unittest.cc
+++ b/payload_state_unittest.cc
@@ -988,6 +988,17 @@
   payload_state.Rollback();
 
   EXPECT_EQ(rollback_version, payload_state.GetRollbackVersion());
+
+  // Change it up a little and verify we load it correctly.
+  rollback_version = "2345.0.1";
+  // Let's verify we can reload it correctly.
+  EXPECT_CALL(*mock_powerwash_safe_prefs, GetString(
+      kPrefsRollbackVersion, _)).WillOnce(DoAll(
+          SetArgumentPointee<1>(rollback_version), Return(true)));
+  EXPECT_CALL(*mock_powerwash_safe_prefs, SetString(kPrefsRollbackVersion,
+                                                    rollback_version));
+  payload_state.LoadRollbackVersion();
+  EXPECT_EQ(rollback_version, payload_state.GetRollbackVersion());
 }
 
 TEST(PayloadStateTest, DurationsAreCorrect) {