histograms: register new CrOS watchdog event

This is used whenever the watchdog service (daisydog) fails to start up
correctly.  We don't expect this to happen on real devices, so we want
to get a handle if it actually does (and how often).

BUG=673501

Review-Url: https://codereview.chromium.org/2571873002
Cr-Commit-Position: refs/heads/master@{#438643}

tools/metrics/histograms/histograms.xml

@@ -44819,6 +44819,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Platform.CrOSEvent" enum="CrosEventEnum">
   <owner>dkrahn@chromium.org</owner>
   <owner>jwerner@chromium.org</owner>
+  <owner>vapier@gentoo.org</owner>
   <summary>
     Generic event of interest from Chrome OS.  Intended mainly to help assess
     the frequency of rare error conditions.
@@ -80505,6 +80506,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="17" label="VeyronEmmcUpgrade.FailedDiskAccess"/>
   <int value="18" label="VeyronEmmcUpgrade.FailedWPEnable"/>
   <int value="19" label="VeyronEmmcUpgrade.SignatureDetected"/>
+  <int value="20" label="Watchdog.StartupFailed"/>
 </enum>
 
 <enum name="CrosFirstRunTutorialCompletionType" type="int">