Delay expiry of PageLoad.Cpu.TotalUsage* histograms

These two page-level metrics are still useful for tracking ads behavior
in Chrome.

Bug: 1138426
Change-Id: Ia279c184383197084af2a0b05ea2069795f3485d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542546Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828210}

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

@@ -695,7 +695,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
-<histogram name="PageLoad.Cpu.TotalUsage" units="ms" expires_after="2021-04-04">
+<histogram name="PageLoad.Cpu.TotalUsage" units="ms" expires_after="2021-11-18">
   <owner>alexmt@chromium.org</owner>
   <owner>johnidel@chromium.org</owner>
   <summary>
@@ -707,7 +707,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="PageLoad.Cpu.TotalUsageForegrounded" units="ms"
-    expires_after="2020-11-18">
+    expires_after="2021-11-18">
   <owner>alexmt@chromium.org</owner>
   <owner>johnidel@chromium.org</owner>
   <summary>