ChromiumOS histograms: Add Cryptohome.DeletedUserProfiles histogram.

In low disk space scenarios, Cryptohome erases whole user profiles.
We count the number of user profiles, deleted during cleanup.

BUG=835114

Change-Id: I8921a4d4c7c8e843c7408b8e1eb7d78b1e8d0785
Reviewed-on: https://chromium-review.googlesource.com/1029753
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554306}

tools/metrics/histograms/histograms.xml

@@ -13551,6 +13551,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Cryptohome.DeletedUserProfiles" units="profiles">
+  <owner>loyso@chromium.org</owner>
+  <summary>
+    In low disk space scenarios, Cryptohome erases whole user profiles. This is
+    a number of user profiles deleted during cleanup. Only reported when
+    non-zero.
+  </summary>
+</histogram>
+
 <histogram name="Cryptohome.DircryptoMigrationEndStatus"
     enum="DircryptoMigrationEndStatus">
   <owner>dspaid@chromium.org</owner>