Extend various security histograms

Bumps the expiration for the Security.SecurityLevel.DownloadStarted,
Security.SecurityLevel.FormSubmission, and Security.PageInfo.TimeOpen
metrics for another six months.

Fixed: 1160748,1160749,1160769,1160773,1160774,1160775,1160776,1160777,1160778
Change-Id: If251de68098a0009b710dc941540e8b4657ad897
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644069
Commit-Queue: Chris Thompson <cthomp@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Auto-Submit: Chris Thompson <cthomp@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845927}

tools/metrics/histograms/histograms_xml/security/histograms.xml

@@ -185,7 +185,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Security.PageInfo.TimeOpen" units="units"
-    expires_after="2021-02-01">
+    expires_after="2021-08-01">
   <owner>cthomp@chromium.org</owner>
   <owner>security-enamel@chromium.org</owner>
   <summary>
@@ -518,7 +518,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Security.SecurityLevel.DownloadStarted" enum="SecurityLevel"
-    expires_after="2021-02-01">
+    expires_after="2021-08-01">
   <owner>cthomp@chromium.org</owner>
   <owner>security-enamel@chromium.org</owner>
   <summary>
@@ -532,7 +532,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Security.SecurityLevel.FormSubmission" enum="SecurityLevel"
-    expires_after="2021-02-01">
+    expires_after="2021-08-01">
   <owner>carlosil@chromium.org</owner>
   <owner>cthomp@chromium.org</owner>
   <summary>