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.h

 // Copyright (c) 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Safe Browsing preferences and some basic utility functions for using them.
 
 #ifndef COMPONENTS_SAFE_BROWSING_DB_SAFE_BROWSING_PREFS_H_
 #define COMPONENTS_SAFE_BROWSING_DB_SAFE_BROWSING_PREFS_H_
 
 #include "base/feature_list.h"
 
 class PrefService;
 
 namespace prefs {
 // Boolean that tell us whether Safe Browsing extended reporting is enabled.
 extern const char kSafeBrowsingExtendedReportingEnabled[];
 
 // Boolean indicating whether Safe Browsing Scout reporting is enabled, which
 // collects data for malware detection.
 extern const char kSafeBrowsingScoutReportingEnabled[];
 
 // Boolean indicating whether the Scout reporting workflow is enabled. This
 // affects which of SafeBrowsingExtendedReporting or SafeBrowsingScoutReporting
 // is used.
 extern const char kSafeBrowsingScoutGroupSelected[];
+
+// Boolean indicating whether the user has ever seen a security interstitial
+// containing the legacy Extended Reporting opt-in.
+extern const char kSafeBrowsingSawInterstitialExtendedReporting[];
+
+// Boolean indicating whether the user has ever seen a security interstitial
+// containing the new Scout opt-in.
+extern const char kSafeBrowsingSawInterstitialScoutReporting[];
 }
 
 namespace safe_browsing {
 
 // Command-line switch for changing the scout_group_selected preference. Should
 // be set to either 'true' or 'false'. Primarily for testing purposes.
 // TODO: this is temporary (crbug.com/662944)
 extern const char kSwitchForceScoutGroup[];
 
 // When this feature is enabled, the Scout opt-in text will be displayed as of
@@ skipping 98 matching lines @@
                                             bool on_show_pref_value);
 
 // Called to indicate that a security interstitial is about to be shown to the
 // user. This may trigger the user to begin seeing the Scout opt-in text
 // depending on their experiment state.
 void UpdatePrefsBeforeSecurityInterstitial(PrefService* prefs);
 
 }  // namespace safe_browsing
 
 #endif  // COMPONENTS_SAFE_BROWSING_DB_SAFE_BROWSING_PREFS_H_