Downloads UMA : Added missing value to enum

Bug: 855654
Change-Id: I8a4dda5e3e073295bc43235dcd4c2bbca15d79e9
Reviewed-on: https://chromium-review.googlesource.com/1112342Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570245}

tools/metrics/histograms/enums.xml

@@ -10397,6 +10397,7 @@ Called by update_net_error_codes.py.-->
   <int value="5" label="CANCEL"/>
   <int value="6" label="OUT_OF_RETRIES"/>
   <int value="7" label="OUT_OF_RESUMPTIONS"/>
+  <int value="8" label="UPLOAD_TIMEOUT"/>
 </enum>
 
 <enum name="Download.Service.EntryEvent">