[iOS SiriShortcuts] Extend histogram

Extends the IOS.SiriShortcuts.Count histogram by 5 months.
The histogram is periodically used to determine the importance
of Siri Shortcuts in iOS 14 and whether richer integration would
be valuable.

Bug: 1167772
Change-Id: Ia9374d20e2bb550f601949fe1b0352cb5e8fd5cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636295Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#844612}

tools/metrics/histograms/histograms_xml/ios/histograms.xml

@@ -805,7 +805,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="IOS.SiriShortcuts.Count" units="shortcuts"
-    expires_after="2021-02-28">
+    expires_after="2021-07-31">
   <owner>gujen@google.com</owner>
   <owner>sebsg@chromium.org</owner>
   <summary>