Fix histogram using attribute "units" when it should be "enum"

This fixes the presentation of this histogram in the UMA dashboards
where it currently shows as representing a numeric value.

Bug: 794828
Change-Id: I549d7460440435861cb4364f303845cfcac5d2e0
Reviewed-on: https://chromium-review.googlesource.com/1072712Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562033}

tools/metrics/histograms/histograms.xml

@@ -60181,7 +60181,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="OfflinePages.DownloadUI.PrefetchedItemHasThumbnail"
-    units="BooleanAvailable">
+    enum="BooleanAvailable">
   <owner>harringtond@chromium.org</owner>
   <summary>
     Whether or not a thumbnail was provided for a prefetched offline article.