Extend lifetime of Net.WebSocket.CloseCode

The stable leg of the experiment that uses this histogram has been postponed
until next year, so extend its life by 6 months.

BUG=1140054

Change-Id: Ia7348b345141e4a90d9bc23d8efa77263698eb3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2567540
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832692}

tools/metrics/histograms/histograms_xml/net/histograms.xml

@@ -4998,7 +4998,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Net.WebSocket.CloseCode" enum="WebSocketCloseCode"
-    expires_after="2020-12-01">
+    expires_after="2021-07-01">
   <owner>ricea@chromium.org</owner>
   <owner>yhirano@chromium.org</owner>
   <owner>yoichio@chromium.org</owner>