Set the FeedEngagementType histogram to never expire.

This histogram is reaching its expiration date, and we realized that
we will need it as long as we have a feed.  After a quick discussion
with the team, we decided to change it to never expire.

Bug: 1167830
Change-Id: I4740a4f76c6ebc285792d803af66a4aac7dd68c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643361Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Commit-Queue: Peter Williamson <petewil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845893}

tools/metrics/histograms/histograms_xml/content/histograms.xml

@@ -719,7 +719,11 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="ContentSuggestions.Feed.EngagementType"
-    enum="FeedEngagementType" expires_after="2021-03-01">
+    enum="FeedEngagementType" expires_after="never">
+<!-- expires-never: key feature metric.  We will need the Engagement
+       type as long as we have a feed, and it will be removed with the
+       feed code if the feed is ever removed. -->
+
   <owner>petewil@chromium.org</owner>
   <owner>harringtond@chromium.org</owner>
   <owner>feed@chromium.org</owner>

tools/metrics/histograms/histograms_xml/new_tab_page/histograms.xml

@@ -306,7 +306,11 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="NewTabPage.ContentSuggestions.Opened" units="index"
-    expires_after="2021-07-01">
+    expires_after="never">
+<!-- expires-never: key feature metric.  We will need the Opened
+     metric as long as we have a feed, and it will be removed with the
+     feed code if the feed is ever removed. -->
+
   <owner>freedjm@chromium.org</owner>
   <owner>feed@chromium.org</owner>
   <summary>
@@ -396,7 +400,11 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="NewTabPage.ContentSuggestions.Shown" units="index"
-    expires_after="2021-07-01">
+    expires_after="never">
+<!-- expires-never: key feature metric.  We will need the Shown
+       metric as long as we have a feed, and it will be removed with
+       the feed code if the feed is ever removed. -->
+
   <owner>freedjm@chromium.org</owner>
   <owner>feed@chromium.org</owner>
   <summary>