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/omaha_response_handler_action.cc b/omaha_response_handler_action.cc
index 8e77afa..3cf78d3 100644
--- a/omaha_response_handler_action.cc
+++ b/omaha_response_handler_action.cc
@@ -50,9 +50,12 @@
   // Note: policy decision to not update to a version we rolled back from.
   string rollback_version =
       system_state_->payload_state()->GetRollbackVersion();
-  if (!rollback_version.empty() && rollback_version == response.version) {
-    LOG(INFO) << "Received version that we rolled back from. Aborting.";
-    return;
+  if(!rollback_version.empty()) {
+    LOG(INFO) << "Detected previous rollback from version " << rollback_version;
+    if(rollback_version == response.version) {
+      LOG(INFO) << "Received version that we rolled back from. Aborting.";
+      return;
+    }
   }
 
   // All decisions as to which URL should be used have already been done. So,