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

components/safe_browsing_db/safe_browsing_prefs.cc

Index: components/safe_browsing_db/safe_browsing_prefs.cc
diff --git a/components/safe_browsing_db/safe_browsing_prefs.cc b/components/safe_browsing_db/safe_browsing_prefs.cc
index 66c1dbb6e477866287b7ecf8bbc8d27b4d5ad2af..1e5ff213c55f9916ac9703c8cbe2fc6d18a9a9fa 100644
--- a/components/safe_browsing_db/safe_browsing_prefs.cc
+++ b/components/safe_browsing_db/safe_browsing_prefs.cc
@@ -57,6 +57,25 @@ enum ActiveExtendedReportingPref {
   MAX_SBER_PREF
 };
 
+// A histogram for tracking a nullable boolean, which can be false, true or
+// null. These values are written to logs. New enum values can be added, but
+// existing enums must never be renumbered or deleted and reused.
+enum NullableBoolean {
+  NULLABLE_BOOLEAN_FALSE = 0,
+  NULLABLE_BOOLEAN_TRUE = 1,
+  NULLABLE_BOOLEAN_NULL = 2,
+  MAX_NULLABLE_BOOLEAN
+};
+
+NullableBoolean GetPrefValueOrNull(const PrefService& prefs,
+                                   const std::string& pref_name) {
+  if (!prefs.HasPrefPath(pref_name)) {
+    return NULLABLE_BOOLEAN_NULL;
+  }
+  return prefs.GetBoolean(pref_name) ? NULLABLE_BOOLEAN_TRUE
+                                     : NULLABLE_BOOLEAN_FALSE;
+}
+
 // Update the correct UMA metric based on which pref was changed and which UI
 // the change was made on.
 void RecordExtendedReportingPrefChanged(
@@ -279,23 +298,27 @@ void RecordExtendedReportingMetrics(const PrefService& prefs) {
   // These metrics track the Scout transition.
   if (prefs.GetBoolean(prefs::kSafeBrowsingScoutGroupSelected)) {
     // Users in the Scout group: currently seeing the Scout opt-in.
-    UMA_HISTOGRAM_BOOLEAN(
+    UMA_HISTOGRAM_ENUMERATION(
         "SafeBrowsing.Pref.Scout.ScoutGroup.SBER1Pref",
-        prefs.GetBoolean(prefs::kSafeBrowsingExtendedReportingEnabled));
-    UMA_HISTOGRAM_BOOLEAN(
+        GetPrefValueOrNull(prefs, prefs::kSafeBrowsingExtendedReportingEnabled),
+        MAX_NULLABLE_BOOLEAN);
+    UMA_HISTOGRAM_ENUMERATION(
         "SafeBrowsing.Pref.Scout.ScoutGroup.SBER2Pref",
-        prefs.GetBoolean(prefs::kSafeBrowsingScoutReportingEnabled));
+        GetPrefValueOrNull(prefs, prefs::kSafeBrowsingScoutReportingEnabled),
+        MAX_NULLABLE_BOOLEAN);
   } else {
     // Users not in the Scout group: currently seeing the SBER opt-in.
-    UMA_HISTOGRAM_BOOLEAN(
+    UMA_HISTOGRAM_ENUMERATION(
         "SafeBrowsing.Pref.Scout.NoScoutGroup.SBER1Pref",
-        prefs.GetBoolean(prefs::kSafeBrowsingExtendedReportingEnabled));
+        GetPrefValueOrNull(prefs, prefs::kSafeBrowsingExtendedReportingEnabled),
+        MAX_NULLABLE_BOOLEAN);
     // The following metric is a corner case. User was previously in the
     // Scout group and was able to opt-in to the Scout pref, but was since
     // removed from the Scout group (eg: by rolling back a Scout experiment).
-    UMA_HISTOGRAM_BOOLEAN(
+    UMA_HISTOGRAM_ENUMERATION(
         "SafeBrowsing.Pref.Scout.NoScoutGroup.SBER2Pref",
-        prefs.GetBoolean(prefs::kSafeBrowsingScoutReportingEnabled));
+        GetPrefValueOrNull(prefs, prefs::kSafeBrowsingScoutReportingEnabled),
+        MAX_NULLABLE_BOOLEAN);
   }
 }