Extend the expiration of the Net.ExpectCT.EntriesOnLoad metric.

The feature it's meant to help evaluate hasn't been enabled on
pre-release channels yet, let alone on stable, so we'll need it around
a fair bit longer than its original expiration date. We're planning on
enabling the feature on pre-release channels in January, after the
field trial freeze ends.

Bug: 969893
Change-Id: Iee2d0238978050cfb2f108693a6f571758052e3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596127Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837763}

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

@@ -1477,7 +1477,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Net.ExpectCT.EntriesOnLoad" units="entries"
-    expires_after="2021-01-07">
+    expires_after="2021-08-07">
   <owner>mmenke@chromium.org</owner>
   <owner>estark@chromium.org</owner>
   <summary>