[Extensions] Extend histogram's lifetime

This commit moves expire date of the following histogram:
 * Extensions.ExtensionCacheCount
 * Extensions.ExtensionCacheSize

The expire date is now in sync with other histograms owned by our team
(Extensions.Force*).

Bug: 1160813
Change-Id: I172aecec7dc32a0b651ccc99c54e212e386145dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2607311Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Oleg Davydov <burunduk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841001}

tools/metrics/histograms/histograms_xml/extensions/histograms.xml

@@ -1138,7 +1138,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Extensions.ExtensionCacheCount" units="units"
-    expires_after="2021-01-31">
+    expires_after="2021-05-30">
   <owner>burunduk@chromium.org</owner>
   <owner>snijhara@google.com</owner>
   <owner>managed-devices@google.com</owner>
@@ -1149,7 +1149,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Extensions.ExtensionCacheSize" units="MB"
-    expires_after="2021-01-31">
+    expires_after="2021-05-30">
   <owner>burunduk@chromium.org</owner>
   <owner>snijhara@google.com</owner>
   <owner>managed-devices@google.com</owner>