Fix histogram enum doc of GCMRegistrationRequestStatus::SUCCESS.

Unlike documented, SUCCESS values do get reported:
https://uma.googleplex.com/p/chrome/timeline_v2/?sid=0ca95a606db22900d36b9304459651b8

Change-Id: Ia252c90c13c96c24b93f1934b8ec4e6802c4090c
Reviewed-on: https://chromium-review.googlesource.com/1137813
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575216}

tools/metrics/histograms/enums.xml

@@ -21851,7 +21851,7 @@ Called by update_net_error_codes.py.-->
 </enum>
 
 <enum name="GCMRegistrationRequestStatus">
-  <int value="0" label="Success (this is not logged currently)"/>
+  <int value="0" label="Success"/>
   <int value="1" label="Invalid parameters"/>
   <int value="2" label="Invalid sender"/>
   <int value="3" label="Authentication failed"/>