Extend the life of ChromeOS.PlatformVerification histograms

BUG=b:175781791

Change-Id: I70cec4f96accb0b24ae079ab0fab4e6441ecf671
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597523
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@{#839651}

tools/metrics/histograms/histograms_xml/chromeos/histograms.xml

@@ -544,7 +544,7 @@ incoming reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="ChromeOS.PlatformVerification.Available"
-    enum="BooleanAvailable" expires_after="2020-10-04">
+    enum="BooleanAvailable" expires_after="2021-12-21">
   <owner>apronin@chromium.org</owner>
   <owner>cros-hwsec+uma@chromium.org</owner>
   <summary>
@@ -554,7 +554,7 @@ incoming reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="ChromeOS.PlatformVerification.Result"
-    enum="ChromeOSPlatformVerificationResult" expires_after="2020-08-31">
+    enum="ChromeOSPlatformVerificationResult" expires_after="2021-12-21">
   <owner>apronin@chromium.org</owner>
   <owner>cros-hwsec+uma@chromium.org</owner>
   <summary>