Change unit for ReadingList histogram

BUG=651041

Review-Url: https://codereview.chromium.org/2576353004
Cr-Commit-Position: refs/heads/master@{#439438}

tools/metrics/histograms/histograms.xml

@@ -51467,7 +51467,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Final status of the download of a reading list entry.</summary>
 </histogram>
 
-<histogram name="ReadingList.FirstReadAgeOnDeletion" units="minutes">
+<histogram name="ReadingList.FirstReadAgeOnDeletion" units="hours">
   <owner>gambard@chromium.org</owner>
   <summary>
     Time since the first read of the reading list entry getting deleted. 0 if it