Update expiry of PageLoad.Clients.Ads...UserActivation histograms

These histograms are still used for tracking regressions for heavy
ad intervention.

Bug: 1156208,1156210,1156211,1156212
Change-Id: I17067bfffa6b8bb143bae85ce475ef37a80fa7e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622318Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843722}

tools/metrics/histograms/histograms_xml/page/histograms.xml

@@ -1305,7 +1305,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="PageLoad.FrameCounts.AdFrames.PerFrame.UserActivation"
-    enum="AdUserActivationStatus" expires_after="2021-01-17">
+    enum="AdUserActivationStatus" expires_after="2021-10-01">
   <owner>johnidel@chromium.org</owner>
   <owner>csharrison@chromium.org</owner>
   <summary>