Extend the life of Platform.MountEncrypted histograms

BUG=b:175781791

Change-Id: If77a2e480689bc49074deeacaa8c054a8705a1af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2599559
Auto-Submit: Yi Chou <yich@google.com>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Yi Chou <yich@google.com>
Cr-Commit-Position: refs/heads/master@{#839684}

tools/metrics/histograms/histograms_xml/platform/histograms.xml

@@ -703,7 +703,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Platform.MountEncrypted.EncryptionKeyStatus"
-    enum="MountEncryptedEncryptionKeyStatus" expires_after="M85">
+    enum="MountEncryptedEncryptionKeyStatus" expires_after="2021-12-21">
   <owner>apronin@chromium.org</owner>
   <owner>mnissler@chromium.org</owner>
   <owner>cros-hwsec+uma@chromium.org</owner>
@@ -715,7 +715,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Platform.MountEncrypted.SystemKeyStatus"
-    enum="MountEncryptedSystemKeyStatus" expires_after="M85">
+    enum="MountEncryptedSystemKeyStatus" expires_after="2021-12-21">
   <owner>apronin@chromium.org</owner>
   <owner>mnissler@chromium.org</owner>
   <owner>cros-hwsec+uma@chromium.org</owner>