update_engine: Add UMA stats for rollback

Adding the following UMA stats:
- UpdateEngine.Check.TargetVersion: first section of the Chrome OS
  target version set by device policy. Sent during update checks if
  target version policy is set.
- UpdateEngine.Check.RollbackTargetVersion: first section of the
  Chrome OS target version if rollback is also allowed. Sent during
  update checks if both policies are set.
- UpdateEngine.EnterpriseRollback.Success: first section of the
  Chrome OS version to which rollback succeeded. Sent after
  successful installation of the rollback image.
- UpdateEngine.EnterpriseRollback.Failure: first section of the
  Chrome OS version to which rollback failed. Sent after
  installation of the rollback image failed.

Chromium CL of the new histograms: crrev.com/c/1069129

BUG=chromium:843622
TEST='cros_run_unit_tests --board=caroline --packages update_engine'

Change-Id: I0b76fa286498ae0a1830a90034734ed9aa5efd3d
Reviewed-on: https://chromium-review.googlesource.com/1062033
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
diff --git a/omaha_response_handler_action.h b/omaha_response_handler_action.h
index e868b53..f5cc1a6 100644
--- a/omaha_response_handler_action.h
+++ b/omaha_response_handler_action.h
@@ -89,6 +89,10 @@
   friend class OmahaResponseHandlerActionTest;
 
   FRIEND_TEST(UpdateAttempterTest, CreatePendingErrorEventResumedTest);
+  FRIEND_TEST(UpdateAttempterTest, RollbackMetricsNotRollbackFailure);
+  FRIEND_TEST(UpdateAttempterTest, RollbackMetricsNotRollbackSuccess);
+  FRIEND_TEST(UpdateAttempterTest, RollbackMetricsRollbackFailure);
+  FRIEND_TEST(UpdateAttempterTest, RollbackMetricsRollbackSuccess);
   FRIEND_TEST(UpdateAttempterTest, SetRollbackHappenedNotRollback);
   FRIEND_TEST(UpdateAttempterTest, SetRollbackHappenedRollback);
   FRIEND_TEST(UpdateAttempterTest, UpdateDeferredByPolicyTest);