Add UMA histograms for enterprise rollback

Adding the following UMA histograms:
- UpdateEngine.Check.TargetVersion: first section of the Chrome OS
  target version set by device policy.
- UpdateEngine.Check.RollbackTargetVersion: first section of the
  Chrome OS target version if rollback is also allowed.
- UpdateEngine.EnterpriseRollback.Success: first section of the
  Chrome OS version to which rollback succeeded.
- UpdateEngine.EnterpriseRollback.Failure: first section of the
  Chrome OS version to which rollback failed.

Update_engine CL: crrev.com/c/1062033

BUG=843622

Change-Id: Ieb30b0bdc965be91c48e85fcf991a1a9116eb5e2
Reviewed-on: https://chromium-review.googlesource.com/1069129
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Marton Hunyady <hunyadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562631}

tools/metrics/histograms/enums.xml

@@ -45892,6 +45892,11 @@ Full version information for the fingerprint enum values:
   <int value="4" label="Reboot Pending"/>
 </enum>
 
+<enum name="UpdateEngineChromeOsVersionPrefix">
+  <int value="-1" label="Invalid"/>
+  <int value="0" label="Not set"/>
+</enum>
+
 <enum name="UpdateEngineConnectionType">
   <int value="0" label="Unknown"/>
   <int value="1" label="Ethernet"/>

tools/metrics/histograms/histograms.xml

@@ -101456,6 +101456,42 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="UpdateEngine.Check.RollbackTargetVersion"
+    enum="UpdateEngineChromeOsVersionPrefix">
+  <owner>hunyadym@chromium.org</owner>
+  <summary>
+    First section of the Chrome OS target version set by device policy and sent
+    to Omaha if rollback is also allowed. This is a sparse histogram, since the
+    possible valid values are stable Chrome OS version prefixes (usually one per
+    milestone).
+
+    Example value: 10323 (meaning M65 stable).
+
+    This is reported on every update check if the TargetVersionPrefix device
+    policy is set and the value of the RollbackToTargetVersion device policy is
+    one of the values allowing rollback.
+
+    This metric is specific to Chrome OS.
+  </summary>
+</histogram>
+
+<histogram name="UpdateEngine.Check.TargetVersion"
+    enum="UpdateEngineChromeOsVersionPrefix">
+  <owner>hunyadym@chromium.org</owner>
+  <summary>
+    First section of the Chrome OS target version set by device policy and sent
+    to Omaha. This is a sparse histogram, since the possible valid values are
+    stable Chrome OS version prefixes (usually one per milestone).
+
+    Example value: 10323 (meaning M65 stable).
+
+    This is reported on every update check if the TargetVersionPrefix device
+    policy is set.
+
+    This metric is specific to Chrome OS.
+  </summary>
+</histogram>
+
 <histogram name="UpdateEngine.Check.TimeSinceLastCheckMinutes" units="minutes">
   <owner>zeuthen@chromium.org</owner>
   <summary>
@@ -101492,6 +101528,40 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="UpdateEngine.EnterpriseRollback.Failure"
+    enum="UpdateEngineChromeOsVersionPrefix">
+  <owner>hunyadym@chromium.org</owner>
+  <summary>
+    First section of the Chrome OS version to which an enterprise rollback
+    (rollback initiated by the admin of the device) has failed. This is a sparse
+    histogram, since the possible valid values are stable Chrome OS version
+    prefixes (usually one per milestone).
+
+    Example value: 10323 (meaning M65 stable).
+
+    This is reported after a rollback image has failed installing.
+
+    This metric is specific to Chrome OS.
+  </summary>
+</histogram>
+
+<histogram name="UpdateEngine.EnterpriseRollback.Success"
+    enum="UpdateEngineChromeOsVersionPrefix">
+  <owner>hunyadym@chromium.org</owner>
+  <summary>
+    First section of the Chrome OS version to which an enterprise rollback
+    (rollback initiated by the admin of the device) has succeeded. This is a
+    sparse histogram, since the possible valid values are stable Chrome OS
+    version prefixes (usually one per milestone).
+
+    Example value: 10323 (meaning M65 stable).
+
+    This is reported after a rollback image has been successfully installed.
+
+    This metric is specific to Chrome OS.
+  </summary>
+</histogram>
+
 <histogram name="UpdateEngine.FailedUpdateCount" units="count">
   <owner>zeuthen@chromium.org</owner>
   <summary>