Make Sber1/2 pref metrics into Nullable Booleans so we can track how often these prefs are unset.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 262ca22a17dafdaaf988794fe64d9b93b08ab987..12edb9c9f6fd1bb14d34397d331647aa1da283d5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -57307,7 +57307,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="SafeBrowsing.Pref.Scout" enum="BooleanEnabled">
+<histogram name="SafeBrowsing.Pref.Scout" enum="NullableBoolean">
   <owner>lpz@chromium.org</owner>
   <summary>
     Tracks the Extended Reporting preference transition. Suffixes track which
@@ -102622,6 +102622,15 @@ from previous Chrome versions.
   <int value="0" label="0x00000000 - STATUS_SUCCESS"/>
 </enum>
 
+<enum name="NullableBoolean" type="int">
+  <summary>
+    A Nullable Boolean can True, False or Null (ie: unset or absent).
+  </summary>
+  <int value="0" label="False"/>
+  <int value="1" label="True"/>
+  <int value="2" label="Null"/>
+</enum>
+
 <enum name="OAuth2LoginAccountRevokedMigrationState" type="int">
   <int value="0" label="Account ID migration not started"/>
   <int value="1" label="Account ID migration in progress"/>