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/mock_metrics_reporter.h b/mock_metrics_reporter.h
index a4e0a12..d080ce8 100644
--- a/mock_metrics_reporter.h
+++ b/mock_metrics_reporter.h
@@ -17,6 +17,8 @@
 #ifndef UPDATE_ENGINE_MOCK_METRICS_REPORTER_H
 #define UPDATE_ENGINE_MOCK_METRICS_REPORTER_H
 
+#include <string>
+
 #include <gmock/gmock.h>
 
 #include "update_engine/metrics_reporter_interface.h"
@@ -29,6 +31,9 @@
 
   MOCK_METHOD1(ReportRollbackMetrics, void(metrics::RollbackResult result));
 
+  MOCK_METHOD2(ReportEnterpriseRollbackMetrics,
+               void(bool success, const std::string& rollback_version));
+
   MOCK_METHOD1(ReportDailyMetrics, void(base::TimeDelta os_age));
 
   MOCK_METHOD4(ReportUpdateCheckMetrics,