Add a new enum for ArcCodeRelocationResult

During OOBE, salt value is empty and boot.art relocation
fails. This is a normal case. Should be reported separately.

Bug: b:72291323
Change-Id: I0bec29d767fb131c0551bf0853b4dc4a62d97d59
Reviewed-on: https://chromium-review.googlesource.com/1152114
Commit-Queue: Xiaoyong Zhou <xzhou@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578714}

tools/metrics/histograms/enums.xml

@@ -1526,6 +1526,7 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="1" label="Error bootlockboxd not ready"/>
   <int value="2" label="Error unable to relocate"/>
   <int value="3" label="Error unable to sign"/>
+  <int value="4" label="Salt empty"/>
 </enum>
 
 <enum name="ArcCodeVerificationResult">