Add metric value to UpdateEngineAttemptResult

Add metric value for kUpdateSkipped.

BUG=chromium:1086956
TEST=None

Change-Id: I864a5ae1aaec0b4936ceba4416ca16b5031e1fff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314807Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Andrew Lassalle <andrewlassalle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791380}

tools/metrics/histograms/enums.xml

@@ -70096,6 +70096,7 @@ Full version information for the fingerprint enum values:
   <int value="10" label="Abnormal Termination"/>
   <int value="11" label="Update Cancelled"/>
   <int value="12" label="Update Succeeded But Not Active"/>
+  <int value="13" label="Skipped Current Update"/>
 </enum>
 
 <enum name="UpdateEngineCertificateCheckStatus">