enums.xml: Add cryptohome event to enums.xml.

Bug: 728125
Change-Id: Id13216db97fafe39ba9aa811401b3071e5213572
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145852Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Greg Kerr <kerrnel@chromium.org>
Auto-Submit: Greg Kerr <kerrnel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758257}

tools/metrics/histograms/enums.xml

@@ -12713,6 +12713,7 @@ to ensure that the crash string is shown properly on the user-facing crash UI.
   <int value="25" label="Crash.Chrome.CrashesFromKernel"/>
   <int value="26" label="Crash.Chrome.MissedCrashes"/>
   <int value="27" label="Crash.Collector.CollectionCount"/>
+  <int value="28" label="Cryptohome.DoubleMountRequest"/>
 </enum>
 
 <enum name="CrosFirstRunTutorialCompletionType">