[Sync::nigori] Extend metrics deadlines to M81.

We still hasn't launched and I can't delete the metrics before seeing
everything being ok. So let's extend them further.

Bug: 988759
Change-Id: Ia24639ebc5636fb55a6acc8b290b3cc3bd6db21b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728651
Auto-Submit: vitaliii <vitaliii@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682820}

tools/metrics/histograms/histograms.xml

@@ -134893,7 +134893,7 @@ should be kept until we use this API.  -->
 
 <histogram
     name="Sync.Crypto.CustomPassphraseKeyDerivationMethodOnNewPassphrase"
-    enum="SyncCustomPassphraseKeyDerivationMethodState" expires_after="M79">
+    enum="SyncCustomPassphraseKeyDerivationMethodState" expires_after="M81">
   <owner>vitaliii@chromium.org</owner>
   <owner>treib@chromium.org</owner>
   <summary>
@@ -134906,7 +134906,7 @@ should be kept until we use this API.  -->
 
 <histogram
     name="Sync.Crypto.CustomPassphraseKeyDerivationMethodOnSuccessfulDecryption"
-    enum="SyncCustomPassphraseKeyDerivationMethodState" expires_after="M79">
+    enum="SyncCustomPassphraseKeyDerivationMethodState" expires_after="M81">
   <owner>vitaliii@chromium.org</owner>
   <owner>treib@chromium.org</owner>
   <summary>
@@ -134919,7 +134919,7 @@ should be kept until we use this API.  -->
 </histogram>
 
 <histogram name="Sync.Crypto.CustomPassphraseKeyDerivationMethodStateOnStartup"
-    enum="SyncCustomPassphraseKeyDerivationMethodState" expires_after="M79">
+    enum="SyncCustomPassphraseKeyDerivationMethodState" expires_after="M81">
   <owner>vitaliii@chromium.org</owner>
   <owner>treib@chromium.org</owner>
   <summary>