Update accessibility histogram expiration dates by 6 months.

As requested by PgM and TLM.

AX-Relnotes: N/A
Bug: 1121204, 1160728
Change-Id: I47c20f62c2ea236d37f2a2537d659fc200069eb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638007Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846237}

tools/metrics/histograms/histograms_xml/accessibility/histograms.xml

@@ -170,7 +170,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Accessibility.CrosAutoclick.TrayMenu.ChangeAction"
-    enum="AutoclickActionType" expires_after="2020-10-04">
+    enum="AutoclickActionType" expires_after="2021-08-19">
   <owner>katie@chromium.org</owner>
   <owner>dtseng@chromium.org</owner>
   <owner>dmazzoni@chromium.org</owner>

tools/metrics/histograms/histograms_xml/dom/histograms.xml

@@ -147,7 +147,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="DomDistiller.Time.ActivelyViewingArticleBeforeDistilling"
-    units="ms" expires_after="2021-02-01">
+    units="ms" expires_after="2021-08-19">
   <owner>katie@chromium.org</owner>
   <owner>chrome-a11y-core@chromium.org</owner>
   <summary>
@@ -161,7 +161,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="DomDistiller.Time.ActivelyViewingReaderModePage" units="ms"
-    expires_after="2021-06-06">
+    expires_after="2021-08-19">
   <owner>katie@chromium.org</owner>
   <owner>chrome-a11y-core@chromium.org</owner>
   <summary>